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>