usbtuner: fix NPE by unplugging Avermedia tuner device

usbtuner: fix NPE by unplugging Avermedia tuner device

Bug: 21806987
Bug 1736911
Bug 2190055

Change-Id: I519a5c544266ed4a0cdf01a84f510629d5ec41ed
Signed-off-by: Terry Heo <terryheo@google.com>
Reviewed-on: http://git-master/r/1311290
(cherry picked from commit 9f6dc884345e1c6fab01fa62d013d8c43e00457f)
Signed-off-by: Magdalena Grodzinska <mgrodzinska@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1806529
Reviewed-by: Aly Hirani <ahirani@nvidia.com>
Reviewed-by: Patrick Horng <phorng@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Vinayak Pane <vpane@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>