[MIPS] Make entry.S a little more readable.

When CONFIG_PREEMPT is not set, it also moves one branch instruction from
ret_from_irq() to ret_from_exception().  Therefore we favour the return
from irq case which should be more common than the other one.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Acked-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>