usb: dwc3: gadget: return error if command sent to DEPCMD register fails

We need to return error to caller if command is not sent to
controller succesfully.

Signed-off-by: Subbaraya Sundeep Bhatta <sbhatta@xilinx.com>
Fixes: 72246da4 (usb: Introduce DesignWare USB3 DRD Driver)
Cc: <stable@vger.kernel.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>