usb: phy: Avoid unchecked dereference warning

Move the USB phy NULL checking before issuing usb_phy_set_charger_current()
to avoid unchecked dereference warning.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>