powerpc/kernel: Fix size of NUM_CPU_FTR_KEYS on 32-bit

The number of CPU feature keys is meant to map 1:1 to the number of CPU
feature flags defined in cputable.h, and the latter must fit in an
unsigned long.

In commit 4db73271 ("powerpc: Add option to use jump label for
cpu_has_feature()"), I incorrectly defined NUM_CPU_FTR_KEYS to 64.

There should be no real adverse consequences of this bug, other than us
allocating too many keys.

Fix it by using BITS_PER_LONG.

Fixes: 4db73271 ("powerpc: Add option to use jump label for cpu_has_feature()")
Tested-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>