nfp: add support for .set_link_ksettings()

Support setting link speed and autonegotiation through
set_link_ksettings() ethtool op.  If the port is reconfigured
in incompatible way and reboot is required the netdev will get
unregistered and not come back until user reboots the system.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>