net: bcmgenet: rename bcmgenet_hw_params->bds_cnt and GENET_DEFAULT_BD_CNT

bcmgenet_hw_params->bds_cnt and GENET_DEFAULT_BD_CNT are used only in Tx init.
Rename them accordingly:
- bcmgenet_hw_params->bds_cnt => bcmgenet_hw_params->tx_bds_per_q
- GENET_DEFAULT_BD_CNT => GENET_Q16_TX_BD_CNT

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