tg3: Move per-int rx members to per-int struct

This patch moves the rx_rcb, rx_rcb_mapping, and rx_rcb_ptr return ring
device members to a per-interrupt structure.  It also adds a new return
ring consumer mailbox register member (consmbox) 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>