powerpc/powermac: Fix unused function warning

powerpc/powermac: Fix unused function warning

Clang reports:
  arch/powerpc/platforms/powermac/feature.c:137:19: error: unused function 'simple_feature_tweak'

It's only used inside the #ifndef CONFIG_PPC64 block, so move it in
there to fix the warning. While at it drop the inline, the compiler will
decide whether it should be inlined or not.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308181501.AR5HMDWC-lkp@intel.com/
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230821140949.491881-1-mpe@ellerman.id.au