usb: gadget: pch_udc: Fix wrong return value

ISSUE:
If the return value of pch_udc_pcd_init() is False, the return value of
this function is unsettled.
Since pch_udc_pcd_init() always returns 0, there is not actually the issue.

CAUSE:
If pch_udc_pcd_init() is True, the variable, retval, is not set for an
appropriate value.

Signed-off-by: Tomoya MORINAGA <tomoya.rohm@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>