ARM: perf: fix compiler warning with gcc 4.6.4 (and tidy code)

GCC 4.6.4 spits out the following warning when building perf_event_v7.c:

arch/arm/kernel/perf_event_v7.c: In function 'krait_pmu_get_event_idx':
arch/arm/kernel/perf_event_v7.c:1927:6: warning: 'bit' may be used uninitialized in this function

While upgrading the version of gcc may solve this, the code can also be
organised to be more efficient by not carrying more local variables than
is necessary across the armv7pmu_get_event_idx function call.  If we set
'bit' to -1 (which is invalid for clear_bit) we can use that as an
indication whether we need to clear a bit after this function.

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