staging: dwc2: cleanups in dwc2_hcd_qh_add()

I've made the success and error paths clearer and pulled some code in
one indent level.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>