selftests/powerpc: Avoid GCC 12 uninitialised variable warning

GCC 12 thinks that `actual` might be used uninitialised. It's not, the
use is guarded by `bad_mmcr2` which is only set to true at the same
point where `actual` is initialised.

  cycles_with_mmcr2_test.c: In function ‘cycles_with_mmcr2’:
  cycles_with_mmcr2_test.c:81:17: error: ‘actual’ may be used uninitialized [-Werror=maybe-uninitialized]
     81 |                 printf("Bad MMCR2 value seen is 0x%lx\n", actual);

Silence the warning by initialising `actual` to zero.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220801113746.802046-1-mpe@ellerman.id.au