smsc75xx: Add workaround for gigabit link up hardware errata.

In certain conditions, the device may not be able to link in gigabit mode. This software workaround ensures that the device will not enter the failure state.

Fixes: d0cad871 ("SMSC75XX USB 2.0 Gigabit Ethernet Devices")
Signed-off-by: Yuiko Oshino <yuiko.oshino@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>