ipmi: allow shared interrupts

The IPMI driver used enable_irq and disable_irq when it got into situations
where it couldn't allocate memory; it did this to avoid having the interrupt
just lock the machine when it couldn't get memory to perform the transaction
to disable the interrupt.

This patch modifies the driver to not use disable_irq and enable_irq.  It
instead sends the messages to the BMC to perform this operation.  It also
makes sure interrupts are cleanly disabled when the interface is shut down and
cleans up some shutdown things that are no longer necessary.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>