USB: gadget: u_serial: free read/write requests upon queue failure

serial driver is dropping the spinlock while queuing the requests and
accessing the port_usb pointer to free the requests upon queue failure.
cable disconnect interrupt can make port_usb NULL which leads to crash
in start_io function while freeing the queue failed requests.
check the port_usb pointer in both RX/TX path and free the queue
failed request immediatly which fixes the crash and traversing the
entire list to free the queue failed requests.

CRs-fixed: 269124
Change-Id: I36eea888d9cf209d181c200255dd1a6a592fc6a7
Signed-off-by: Chiranjeevi Velempati <cvelempa@codeaurora.org>