isp1704_charger: remove useless check in isp1704_charger_probe()

Neither devm_usb_get_phy_by_phandle() nor devm_usb_get_phy() can
return a NULL result, so remove the useless !isp->phy check.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>