sfc: handle limited FEC support

If the reported PHY capabilities do not include a given FEC mode, don't
 attempt to select that FEC mode anyway.  If the user tries to set a mode
 through ethtool that is not supported, return an error.
The _REQUESTED bits don't appear in the supported caps, but are implied
 by the corresponding FEC bits.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>