usb: f_qdss: Setting channel to NULL in qdss_close

When qdss_close is executed before disconnect_work
there is possibility that from disconnect_work channel
priv would be passed as NULL to the notifier resulting
in page fault error.

Fix this by setting notify to NULL on qdss_close.

Change-Id: Ifdfc43733221fdc1dc3bcd8a40f32e1b021398a8
Signed-off-by: Udipto Goswami <ugoswami@codeaurora.org>