net: phy: LAN937x: added PHY_POLL_CABLE_TEST flag

Added the phy_poll_cable_test flag for the lan937x phy driver.
Tested using command -  ethtool --cable-test <dev>

Fixes: 680baca5

 ("net: phy: added the LAN937x phy support")
Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>