usb: phy: mxs: Staticize mxs_charger_secondary_detection()

mxs_charger_secondary_detection() is only used in this file, so make
it static.

This fixes the following sparse warning:

drivers/usb/phy/phy-mxs-usb.c:581:23: warning: symbol 'mxs_charger_secondary_detection' was not declared. Should it be static?
Acked-by: Jun Li <jun.li@nxp.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/phy/phy-mxs-usb.c

@@ -578,7 +578,7 @@ static enum usb_charger_type mxs_charger_primary_detection(struct mxs_phy *x)
  * It must be called after DP is pulled up, which is used to
  * differentiate DCP and CDP.
  */
-enum usb_charger_type mxs_charger_secondary_detection(struct mxs_phy *x)
+static enum usb_charger_type mxs_charger_secondary_detection(struct mxs_phy *x)
 {
 	struct regmap *regmap = x->regmap_anatop;
 	int val;