usb: gadget: u_serial: Don't dequeue requests in gserial_disconnect

The following patches are reverted since dequeuing all requests
in gserial_disconnect() with interrupts disabled is resulting
in stability issues. The original problem of end transfer timeout
in DWC3 driver is not completely solved with dequeuing the requests,
so this patch does not introduce any regressions.

f3314511 usb: gadget: u_serial: Remove extra list operation from
gs_start_tx.

fb8bceab usb: gadget: u_serial: Rectify the list operations is
rx/tx path.

83626bc1 usb: gadget: u_serial: Dequeue request on gserial_disconnect.

Change-Id: Ic8a8cbaf295d1cb335b463743814a289c89069b8
Signed-off-by: Uttkarsh Aggarwal <quic_uaggarwa@quicinc.com>