Input: serio - take drv_mutex in serio_cleanup()

We need to take serio->drv_mutex in serio_cleanup() to prevent the
function from being called while driver is in the middle of attaching
to a serio port. Such situation can happen with i8042 and atkbd drivers
if user rapidly presses Ctrl-Alt-Del during system startup, and leads
to kernel oops.

Reported-by: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>