powerpc: Fix usage of 64-bit instruction in 32-bit altivec code

e821ea70

 introduced a bug by copying
some 64-bit originated code as-is to be used by both 32 and 64-bit
but this code contains a 64-bit ony "cmpdi" instruction.

This changes it to cmpwi, which is fine since VRSAVE can only contains
a 32-bit value anyway.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: <stable@kernel.org>