tg3: Add mailbox assignments

The 5717 assigns mailbox locations to interrupt vectors in a rather
non-intuitive way.  (Much of the complexity stems from legacy
compatibility issues.)  This patch implements the assignment scheme.

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