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>