net/mlx4_core: Deprecate use_prio module parameter

use_prio was added as part of an infrastructure for running FCoE in A0 mode.
FCoE didn't get into Mellanox Upstream driver, and when it will, it won't be
using A0 steering mode.

Therefore we can safely deprecate this module parameter without hurting any
existing user.

CC: Carol Soto <clsoto@linux.vnet.ibm.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>