usb: gadget: Remove usb_diag_free_req() API

Freeing USB requests either as part of cable disconnect or in unbind
case, both doesn't depends on diag driver to initiate usb free request
using usb_diag_free_req(). Freeing of USB request as part of cable
disconnect is handled in the code by freeing the USB requests in next
cable connect. In unbind case, usb requests are freed in usb driver
before releasing reference to usb diag device context.

Although freeing of usb request is taken care in usb driver, but it
is seen in few regression testing that calling usb_diag_free_req()
from diag is leading to synchronization issues. As USB doesn't depend
on the diag driver to initiate usb free request using API
usb_diag_free_req(), hence remove API usb_diag_free_req() from code.

CRs-Fixed: 583730
Change-Id: I990f6c07ef28e767040cc959d0addf8ab413d1d1
Signed-off-by: Saket Saurabh <ssaurabh@codeaurora.org>