ARC: perf: redo the pct irq missing in device-tree handling

commit feb92d7d "(ARC: perf: don't bail setup if pct irq
missing in device-tree)" introduced a silly brown-paper bag bug:
The assignment and comparison in an if statement were not bracketed
correctly leaving the order of evaluation undefined.

|
| if (has_interrupts && (irq = platform_get_irq(pdev, 0) >= 0)) {
|                           ^^^                         ^^^^

And given such a chance, the compiler will bite you hard, fully entitled
to generating this piece of beauty:

|
| # if (has_interrupts && (irq = platform_get_irq(pdev, 0) >= 0)) {
|
| bl.d @platform_get_irq  <-- irq returned in r0
|
| setge r2, r0, 0   	<-- r2 is bool 1 or 0 if irq >= 0 true/false
| brlt.d r0, 0, @.L114
|
| st_s	r2,[sp]    	<-- irq saved is bool 1 or 0, not actual return val
| st	1,[r3,160]   	# arc_pmu.18_29->irq <-- drops bool and assumes 1
|
| # return __request_percpu_irq(irq, handler, 0,
|
| bl.d @__request_percpu_irq;
| mov_s	r0,1	   <-- drops even bool and assumes 1 which fails

With the snafu fixed, everything is as expected.

| bl.d @platform_get_irq	<-- returns irq in r0
|
| mov_s	r2,r0
| brlt.d r2, 0, @.L112
|
| st_s	r0,[sp]			<-- irq isaved is actual return value above
| st	r0,[r13,160]	#arc_pmu.18_27->irq
|
| bl.d @__request_percpu_irq	<-- r0 unchanged so actual irq returned
| add r4,r4,r12	#, tmp363, __ptr

Cc: <stable@vger.kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/kernel/perf_event.c

@@ -562,7 +562,7 @@ static int arc_pmu_device_probe(struct platform_device *pdev)
 {
 	struct arc_reg_pct_build pct_bcr;
 	struct arc_reg_cc_build cc_bcr;
-	int i, has_interrupts, irq;
+	int i, has_interrupts, irq = -1;
 	int counter_size;	/* in bits */
 
 	union cc_name {
@@ -637,19 +637,28 @@ static int arc_pmu_device_probe(struct platform_device *pdev)
 		.attr_groups	= arc_pmu->attr_groups,
 	};
 
-	if (has_interrupts && (irq = platform_get_irq(pdev, 0) >= 0)) {
+	if (has_interrupts) {
+		irq = platform_get_irq(pdev, 0);
+		if (irq >= 0) {
+			int ret;
 
 			arc_pmu->irq = irq;
 
 			/* intc map function ensures irq_set_percpu_devid() called */
-		request_percpu_irq(irq, arc_pmu_intr, "ARC perf counters",
+			ret = request_percpu_irq(irq, arc_pmu_intr, "ARC perf counters",
 						 this_cpu_ptr(&arc_pmu_cpu));
 
+			if (!ret)
 				on_each_cpu(arc_cpu_pmu_irq_init, &irq, 1);
-	} else {
-		arc_pmu->pmu.capabilities |= PERF_PMU_CAP_NO_INTERRUPT;
+			else
+				irq = -1;
+		}
+
 	}
 
+	if (irq == -1)
+		arc_pmu->pmu.capabilities |= PERF_PMU_CAP_NO_INTERRUPT;
+
 	/*
 	 * perf parser doesn't really like '-' symbol in events name, so let's
 	 * use '_' in arc pct name as it goes to kernel PMU event prefix.