[MIPS] Remove set_c0_status(ST0_IM) from wrppmc's irq.c.

mips_cpu_irq_init() does clear_c0_status(ST0_IM) first, so
set_c0_status(ST0_IM) isn't necessary.

Signed-off-by: Yoichi Yuasa <yoichi_yuasa@tripeaks.co.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>