Input: elantech - packet checking for v2 hardware

For v2 hardware, there is no real parity check, but we can still check
some constant bits for data integrity.

Also rename elantech_check_parity_v1 to elantech_packet_check_v1 to make
these packet checking function names consistent.

Signed-off-by: JJ Ding <jj_ding@emc.com.tw>
Acked-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Éric Piel <eric.piel@tremplin-utc.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>