net: bcmgenet: fix Tx ring priority programming

GENET MAC has three Tx ring priority registers:
- GENET_x_TDMA_PRIORITY0 for queues 0-5
- GENET_x_TDMA_PRIORITY1 for queues 6-11
- GENET_x_TDMA_PRIORITY2 for queues 12-16

Fix bcmgenet_init_multiq() to program them correctly.

Signed-off-by: Petri Gynther <pgynther@google.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>