As all the infrastructure for multiple transmit queues already exists
in the driver, this patch is entirely trivial.

The individual transmit queues are still serialised by the driver's
per-port private spinlock, but that will disappear (i.e. be replaced
by the per-subqueue ->_xmit_lock) in a subsequent patch.
Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>