tg3: Add per-int coalesce now member

Each interrupt vector has its own bit in the host coalescing register to
force that vector's status block to be updated and generate an
interrupt.  This patch adds a member to the per-interrupt structure
that records which bit belongs to that vector.

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