Input: elantech - make sure touchpad is really in absolute mode

There exist laptops with Elantech touchpads where switching to absolute mode
does not happen, although writing the configuration register succeeds
without error. Reading back the register afterwards reveils that the
absolute mode bit is not set as if masked out by the touchpad firmware.

Always read back register 0x10, make sure that for hardware version 1 the
absolute mode bit is actually set and fail otherwise. This prevents the case
where the touchpad is claimed by the Elantech driver but is nonetheless not
working.

Signed-off-by: Arjan Opmeer <arjan@opmeer.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>