usb: gadget: qdss: Add NULL check against channel with usb_qdss_free_req()

USB requests are being freed from 2 context from Linux QDSS driver:
1. usb_qdss_close()
2. explicitly calling usb_free_request() on receiving USB_QDSS_DISCONNECT

usb_qdss_close() is setting drvdata->usbch to NULL. Hence it ends up
passing usbch as NULL while calling usb_free_request() when handling
USB_QDSS_DISCONNECT. Fix this NULL pointer dereference issue by explicitly
checking USB channel against NULL with usb_qdss_free_req().

Change-Id: Ia55a30b09ab1f94db15947b984fd232a33da7788
Signed-off-by: Mayank Rana <mrana@codeaurora.org>