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

This patch moves the last_tag, last_tag_irq, and hw_status device
members to a per-interrupt structure.  It also adds a new interrupt
mailbox member (int_mbox) and converts the code to use it rather than a
direct preprocessor constant.

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