Use an irq_enable_hazard hazard barrier in unmask_mips_irq. This

hasn't been an actual bug, so it's more a change to be 100% compliant
with the requirements of the architecture spec.  Similar fix to
mask_mips_irq where there was a slightly less theoretical chance of
getting hit.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>