xtensa: fix kernel/user jump out of fast_unaligned

Use correct register (a0, just read from the PS) to check user mode bit.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/kernel/align.S

@@ -441,7 +441,7 @@ ENTRY(fast_unaligned)
 	mov	a1, a2
 
 	rsr	a0, ps
-	bbsi.l  a2, PS_UM_BIT, 1f     # jump if user mode
+	bbsi.l  a0, PS_UM_BIT, 1f     # jump if user mode
 
 	movi	a0, _kernel_exception
 	jx	a0