[MIPS] Drop __init from init_8259A()

init_8259A() is called from i8259A_resume() so should not be marked as
__init.  And add some tests for whether 8259A was already initialized
or not.

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