ipmi: Compensate for BMCs that wont set the irq enable bit

It appears that some BMCs support interrupts but don't support setting
the irq enable bits.  The interrupts are just always on.  Sigh.
Add code to compensate.

The new code was very similar to another functions, so this also
factors out the common code into other functions.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Tested-by: Henrik Korkuc <henrik@kirneh.eu>