ethtool/mii support

Bug: disabling autonegotiation and setting the link parameters at the
same time does not provide the expected result. More investigation is
needed.

Note: past the initial probe/open time, the link is managed from user-space
or accessed through sis190_phy_task, i.e. in a usermode context. Whence the
very limited locking needs.
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>