x86: irq.c - tiny cleanup

Impact: cleanup, robustization

 1) guard ack_bad_irq with printk_ratelimit since there is no
    guarantee we will not be flooded one day

 2) use pr_emerg() helper

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
LKML-Reference: <20090412165058.277579847@openvz.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>