There is a downstream patch which adds a new interface type -
PHY_INTERFACE_MODE_2500SGMII (which is really the same one as
PHY_INTERFACE_MODE_2500BASEX).

We backported from upstream the following phylink patch which, of
course, does not treat the PHY_INTERFACE_MODE_2500SGMII interface mode
in a switch case statement.
	34ae2c09 ("net: phylink: add generic validate implementation")

Because of this, we get the following build warning.

drivers/net/phy/phylink.c: In function ‘phylink_get_linkmodes’:
drivers/net/phy/phylink.c:322:2: warning: enumeration value ‘PHY_INTERFACE_MODE_2500SGMII’ not handled in switch [-Wswitch]
  322 |  switch (interface) {
      |  ^~~~~~

Fix it by treating the new interface mode in the switch-case statement.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>