[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: Ralf Baechle <ralf@linux-mips.org>