usb: chipidea: fix return value check in ci_hdrc_pci_probe()

In case of error, the function usb_phy_generic_register()
returns ERR_PTR() and never returns NULL. The NULL test in
the return value check should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Peter Chen <peter.chen@nxp.com>