net: bcmgenet: rework Tx queue init

1. Rename bcmgenet_init_multiq() to bcmgenet_init_tx_queues()
2. Fix bcmgenet_init_tx_queues() function description
3. Move Tx default queue init inside bcmgenet_init_tx_queues()
4. Modify bcmgenet_init_dma() to call bcmgenet_init_tx_queues()

Signed-off-by: Petri Gynther <pgynther@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>