sfc: 10Xpress: Report support for pause frames

Commits 27fbc7db 'mdio: Expose pause frame advertising flags to ethtool'
and c634263d 'sfc: 10Xpress: Initialise pause advertising flags'
added to our reported advertising flags.

efx_mdio_set_settings() requires that all advertising flags are
also present in the supported flags, so make sure that is true.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>