bnx2: Use mutex on slow path cnic calls.

The slow path calls to the cnic driver are sleepable calls so we
cannot use rcu_read_lock().  Use mutex for these slow path calls
instead.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>