Input: serio - convert to common workqueue instead of a thread

Instead of creating an exclusive thread to handle serio events (which
happen rarely), let's switch to using common workqueue. With the arrival
of concurrency-managed workqueue infrastructure we are not concerned
that our callers or callees also using workqueue (no deadlocks anymore)
and it should reduce total number of threads in the system.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>