sfp: add support for 5gbase-t SFPs

The sfp_parse_support() function is setting 5000baseT_Full in some cases.
Now that we have PHY_INTERFACE_MODE_5GBASER interface mode available,
change sfp_select_interface() to return PHY_INTERFACE_MODE_5GBASER if
5000baseT_Full is set in the link mode mask.

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>