ipmi: Fix a memory ordering issue

From a locking point of view it is safe to check waiting_msg without
a lock, but there is a memory ordering issue that causes it to
possibly not be set right when viewed from another processor.  We are
already claiming a lock right after that, move the check to inside
the lock to enforce the memory ordering.

Signed-off-by: Corey Minyard <cminyard@mvista.com>