tg3: Move per-int tx members to a per-int struct

This patch moves the tx_prod, tx_cons, tx_pending, tx_ring, and
tx_buffers transmit ring device members to a per-interrupt structure.
It also adds a new transmit producer mailbox member (prodmbox) and
converts the code to use it rather than a preprocessor constant.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>