[POWERPC] Fix is_power_of_4(x) compile error

When building an 85xx kernel we get:

  CC      arch/powerpc/mm/pgtable_32.o
arch/powerpc/mm/pgtable_32.c: In function 'io_block_mapping':
arch/powerpc/mm/pgtable_32.c:330: error: expected identifier before '(' token
arch/powerpc/mm/pgtable_32.c:330: error: expected statement before ')' token

The is_power_of_2(x) fixup patch left an extra ')' on the is_power_of_4 macro.
There is a similiar issue on the arch/ppc side.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>