USB: Gadget: u_serial: Freeing usb requests as a part of gs_close

The usb requests allocated during gs_open() were freed only as a part of
usb disconnect previously. So freeing the usb requests as a part of
gs_close() helps to free the requests on every network disconnect also.

Change-Id: I6edcab3cba94902ea3cb1a043403c93afa8c9d52
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>