x86: use limited register constraint for setnz

Impact: build fix with certain compilers

GCC can decide to use %dil when "r" is used, which is not valid for
setnz.

This bug was brought out by Stephen Rothwell's merging of the
branch tracer into linux-next.

[ Thanks to Uros Bizjak for recommending 'q' over 'Q' ]
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/boot/tty.c

@@ -74,7 +74,7 @@ static int kbd_pending(void)
 {
 	u8 pending;
 	asm volatile("int $0x16; setnz %0"
-		     : "=rm" (pending)
+		     : "=qm" (pending)
 		     : "a" (0x0100));
 	return pending;
 }