powerpc/dt_cpu_ftrs: Make use of macro ISA_V3_1

Macro ISA_V3_1 was defined but never used.  Use it instead of literal.
Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200610215114.167544-4-muriloo@linux.ibm.com

arch/powerpc/kernel/dt_cpu_ftrs.c

@@ -678,7 +678,7 @@ static void __init cpufeatures_setup_start(u32 isa)
 		cur_cpu_spec->cpu_user_features2 |= PPC_FEATURE2_ARCH_3_00;
 	}
 
-	if (isa >= 3100) {
+	if (isa >= ISA_V3_1) {
 		cur_cpu_spec->cpu_features |= CPU_FTR_ARCH_31;
 		cur_cpu_spec->cpu_user_features2 |= PPC_FEATURE2_ARCH_3_1;
 	}