usb_printer_device: Unplug printer in Reset

If client does not Unplug printer in the end, printer will remain
plugged. It will block g_audio usage.

BUG=chromium:850874
TEST=Call Reset and see g_printer is removed.

Change-Id: I97802f3ef2b34c728b7e192bbb8a171b7b3c81f9
Reviewed-on: https://chromium-review.googlesource.com/1092617
Commit-Ready: Cheng-Yi Chiang <cychiang@chromium.org>
Tested-by: Cheng-Yi Chiang <cychiang@chromium.org>
Reviewed-by: Wai-Hong Tam <waihong@google.com>
1 file changed