be2net: use mutex instead of spin lock for mbox_lock

Since the mbox polling uses the schedule_timeout, the mbox_lock should be
a mutex and not a spin lock.
The commit f25b03a replaced udelay() with schedule_timeout() but didn't
change mbox_lock to semaphore or mutex.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>