tifm_7xx1: switch from workqueue to kthread

As there's only one work item (media_switcher) to handle and it's effectively
serialized with itself, I found it more convenient to use kthread instead of
workqueue. This also allows for a working implementation of suspend/resume,
which were totally broken in the past version.

Signed-off-by: Alex Dubov <oakad@yahoo.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>