Input: serio/gameport - use 'long' system workqueue

Commit 8ee294cd converted serio
subsystem event handling from using a dedicated thread to using
common workqueue. Unfortunately, this regressed our boot times,
due to the fact that serio jobs take long time to execute. While
the new concurrency managed workqueue code manages long-playing
works just fine and schedules additional workers as needed, such
works wreck havoc among remaining users of flush_scheduled_work().

To solve this problem let's move serio/gameport works from system_wq
to system_long_wq which nobody tries to flush.

Reported-and-tested-by: Hernando Torque <pantherchen@versanet.de>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>