ARM: 6469/1: perf-events: squash compiler warning

armv7_pmnc_counter_has_overflowed can return uninitialised data
if an invalid counter is specified.

This patch fixes the code to return 0 in this case, which squashes
the compiler warning from GCC 4.5.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>