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

commit 891b1dc022955d36cf4c0f42d383226a930db7ed upstream.

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: b09bb642 (usb: dwc3: gadget: implement Global Command support)
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>