Input: elantech - allow forcing Elantech protocol

Apparently hardware vendors now ship elantech touchpads with different version
magic. This options allows for them to be tested easier with the current driver
in order to add their magic to the whitelist later.

Signed-off-by: Florian Ragwitz <rafl@debian.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>