usb: gadget: serial: Prevent use-after-free

This change prevents use-after-free of dwc3_requests
during power on/off with USB connected. Since checking
of port->usb means that by that time gserial_disconnect
must have been called and as a part of disconnect we will
free the request pool.

Change-Id: Iee55d9e580e45daaf84e69d89390a3d91b185a80
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>