The signatures of v3 and v4 packets change depending on the value of a
hardware flag called 'crc_enabled'. The packet type detection must change
accordingly.

This patch also restores a consistency check for v4 packets inadvertently
removed by commit:

9eebed7d
Input: elantech - fix for newer hardware versions (v7)

A note about the naming convention: v3 hardware is associated with IC body
v5 while v4 hardware is associated with IC body v6 and v7. The above commit
refers to IC body v7, not to v7 hardware.

Tested on Samsung NP730U3E (fw = 0x675f05, ICv7, crc_enabled = 1)
Tested-by: Giovanni Frigione <gio.frigione@gmail.com>
Signed-off-by: Matteo Delfino <kendatsuba@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>