[MIPS] do_IRQ cleanup

Now we have both function and macro version of do_IRQ() and the former
is used only by DEC and non-preemptive kernel.  This patch makes
everyone use the macro version and removes the function version.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>