usb: dwc3: fix a test for error in dwc3_core_init()

This was refactored recently and the "ret = " was accidentally deleted
so the errors aren't checked.

Fixes: 1d72fab4

 ("USB: dwc3: refactor phy handling")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Johan Hovold <johan+linaro@kernel.org>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/d0c5a04f-deee-4ebe-9b0b-dc5492564519@kili.mountain
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>