usb: gadget: f_ncm: Use atomic_t to track in-flight request

Currently ncm->notify_req is used to flag when a request is in-flight.
ncm->notify_req is set to NULL and when a request completes it is
subsequently reset.

This is fundamentally buggy in that the unbind logic of the NCM driver will
unconditionally free ncm->notify_req leading to a NULL pointer dereference.

Fixes: 40d133d7 ("usb: gadget: f_ncm: convert to new function interface with backward compatibility")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>