[MIPS] SMTC: Don't set and restore irqregs ptr from self_ipi.

This did corrupt register s0 which the caller of self_ipi expects to
be unchanged.  This is a kernel bug which will only be triggered with
the compilers which compile __smtc_ipi_replay to use s0 across the
invocation of self_ipi. Gcc 4.1.2 does this, for example.
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/kernel/smtc-asm.S

@@ -121,10 +121,7 @@ LEAF(self_ipi)
 	subu	t1,sp,PT_SIZE
 	sw	ra,PT_EPC(t1)
 	sw	a0,PT_PADSLOT4(t1)
-	LONG_L	s0, TI_REGS($28)
-	LONG_S	sp, TI_REGS($28)
 	la	t2,ipi_decode
-	LONG_S	s0, TI_REGS($28)
 	sw	t2,PT_PADSLOT5(t1)
 	/* Save pre-disable value of TCStatus */
 	sw	t0,PT_TCSTATUS(t1)