microblaze: Improve checking mechanism for MSR instruction

It is more safe to use clear instead of msrset.
We save some instructions too.
Signed-off-by: NMichal Simek <monstr@monstr.eu>

arch/microblaze/kernel/head.S

@@ -54,19 +54,16 @@ ENTRY(_start)
 	mfs	r1, rmsr
 	andi	r1, r1, ~2
 	mts	rmsr, r1
-
 /*
  * Here is checking mechanism which check if Microblaze has msr instructions
  * We load msr and compare it with previous r1 value - if is the same,
  * msr instructions works if not - cpu don't have them.
  */
-	or	r8, r0, r0 /* 0 - I have msr instr, 1 - I don't have */
-	or	r12, r0, r0
-	msrset	r12, 0	/* set nothing - just read msr for test */
-	cmpu	r12, r12, r1
-	beqi	r12, 1f
-	ori	r8, r0, 1 /* I don't have msr */
-1:
+	/* r8=0 - I have msr instr, 1 - I don't have them */
+	rsubi	r0, r0, 1	/* set the carry bit */
+	msrclr	r0, 0x4		/* try to clear it */
+	/* read the carry bit, r8 will be '0' if msrclr exists */
+	addik	r8, r0, 0
 
 /* r7 may point to an FDT, or there may be one linked in.
    if it's in r7, we've got to save it away ASAP.