sfc: 10Xpress: Initialise pause advertising flags

The mdio module now handles reconfiguration of pause advertising
through ethtool, but not initialisation.  Add the necessary
initialisation to tenxpress_phy_init().

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