powerpc: Correct used_vsr comment

The used_vsr flag is set if process has used VSX registers, not Altivec
registers. But the comment says otherwise, correct the comment.
Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/processor.h

@@ -246,7 +246,7 @@ struct thread_struct {
 #endif /* CONFIG_ALTIVEC */
 #ifdef CONFIG_VSX
 	/* VSR status */
-	int		used_vsr;	/* set if process has used altivec */
+	int		used_vsr;	/* set if process has used VSX */
 #endif /* CONFIG_VSX */
 #ifdef CONFIG_SPE
 	unsigned long	evr[32];	/* upper 32-bits of SPE regs */