tg3: Fix link check in tg3_adjust_link

The tg3 driver tried to detect link changes by comparing the tg3 local
active_speed member with SPEED_UNKNOWN (or formerly SPEED_INVALID).
This check is not correct, since phylib will never set its speed member
to either of these two values.  The code only appeared to work because
tg3 initializes active_speed to SPEED_INVALID during tg3_init_one.  This
patch introduces a new "old_link" tg3 member and then compares the
phy_device's link member against it to detect link state changes.

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