ar9170usb: reset device on resume

This patch takes care of an outstanding comment in
"[PATCH] ar9170usb: fix hang on resume" commit message.

>However, the device does not accept the firmware on resume.
>and it will exit with:
>
>> firmware part 1 upload failed (-71).
>> device is in a bad state. please reconnect it!

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Christian Lamparter <chunkeey@web.de>
Tested-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>