via-rhine: fix full-duplex with autoneg disable

With some specific configuration (VT6105M on Soekris 5510 and depending
on the device at the other end), fragmented packets were not transmitted
when forcing 100 full-duplex with autoneg disable.

This fix now write full-duplex chips register when forcing full or
half-duplex not only when autoneg is enable.

Signed-off-by: François Cachereul <f.cachereul@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>