Input: elantech - enable SMBus on new (2018+) systems

There are some new HP laptops with Elantech touchpad that don't support
multitouch.

Currently we use ETP_NEW_IC_SMBUS_HOST_NOTIFY() to check if SMBus is supported,
but in addition to firmware version, the bus type also informs us whether the IC
can support SMBus. To avoid breaking old ICs, we will only enable SMbus support
based the bus type on systems manufactured after 2018.

Lastly, let's consolidate all checks into elantech_use_host_notify() and use it
to determine whether to use PS/2 or SMBus.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>