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: NBenjamin Herrenschmidt <benh@kernel.crashing.org>
CC: <stable@kernel.org>

arch/powerpc/kernel/vector.S

@@ -58,7 +58,7 @@ _GLOBAL(load_up_altivec)
 	 * all 1's
 	 */
 	mfspr	r4,SPRN_VRSAVE
-	cmpdi	0,r4,0
+	cmpwi	0,r4,0
 	bne+	1f
 	li	r4,-1
 	mtspr	SPRN_VRSAVE,r4