Revert "Input: serio - make write method mandatory"

This reverts commit 81c7c0a3

. The idea
to make write method mandatory was flawed as several client drivers
(such as atkbd) check for presence of write() method to adjust behavior
of the driver.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>