net: bcmgenet: precalculate TxCB->bd_addr

There is 1-to-1 mapping between TxCBs and TxBDs. Precalculate TxCB->bd_addr
once in bcmgenet_init_dma() instead of doing it over and over needlessly in
bcmgenet_get_txcb().

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