usb: renesas_usbhs: gadget: fix return value check in usbhs_mod_gadget_probe()

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

Fixes: b5a28756 ("usb: renesas_usbhs: Allow an OTG PHY driver to
	provide VBUS")
Cc: <stable@vger.kernel.org> # v4.3+
Acked-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>