Input: serio - make write method mandatory

Given that all serio drivers except one implement write() method
let's make it mandatory to avoid testing for its presence whenever
we attempt to use it.

Link: https://lore.kernel.org/r/YFgUxG/TljMuVeQ3@google.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>