MIPS: cevt-r4k: Don't call get_c0_compare_int if timer irq is installed

In the Linux kernel, the following vulnerability has been resolved:

MIPS: cevt-r4k: Don't call get_c0_compare_int if timer irq is installed

This avoids warning:

[ 0.118053] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:283

Caused by get_c0_compare_int on secondary CPU.

We also skipped saving IRQ number to struct clock_event_device *cd as

it's never used by clockevent core, as per comments it's only meant

for "non CPU local devices".