ipmi: Handle BMCs that don't allow clearing the rcv irq bit

Some BMCs don't let you clear the receive irq bit in the global
enables.  This is kind of silly, but they give an error if you
try to clear it.  Compensate for this by detecting the situation
and working around it.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Tested-by: Thomas D <whissi@whissi.de>
Reviewed-by: Thomas D <whissi@whissi.de>

diff --cc drivers/char/ipmi/ipmi_si_intf.c
Simple merge