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

e821ea70f3b4873b50056a1e0f74befed1014c09 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 <[e-mail]>
CC: <[e-mail]>

arch/powerpc/kernel/vector.S

···
 	 * all 1's
 	 */
 	mfspr	r4,SPRN_VRSAVE
-	cmpdi	0,r4,0
+	cmpwi	0,r4,0
 	bne+	1f
 	li	r4,-1
 	mtspr	SPRN_VRSAVE,r4