usb: gadget: qdss: Add NULL check against priv_usb with usb_qdss_close()

In some cases, it has been observed that Coresight QDSS driver
is not callig usb_qdss_open() API before calling usb_qdss_close() API.
This results into NULL pointer dereference related crash within
usb_qdss_close() API as per channel based priv_usb is not being
initialized (it is happening with usb_qdss_open() context). Hence
add explicit check aginst NULL before using ch->priv_usb instead
of depending on client driver.

Change-Id: Ib5073d9262a06c0101d14a23dd6a2fdb77861ff3
Signed-off-by: Mayank Rana <mrana@codeaurora.org>