net: bcmgenet: bcmgenet_init_tx_ring() cleanup

1. Simplify function description
2. Rename function parameter write_ptr to start_ptr to better indicate use
3. Remove unnecessary local variable first_bd
4. Remove out-of-place comment "Unclassified traffic goes to ring 16"
5. Fix TDMA_WRITE_PTR register init

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