qlge: Add/use function for link up/down.

We need to set/clear the mac address register when the link goes up/down
respectively.  Without this both ports of a 2-port device can end up
with the same mac address in a bonding scenario.
The new ql_link_on() and ql_link_off() will also be used in handling
certain firmware events.

Signed-off-by: Ron Mercer <ron.mercer@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>