Input: ALPS - fix multi-touch decoding on SS4 plus touchpads

The fix for handling two-finger scroll (i4a646580f793 - "Input: ALPS -
fix two-finger scroll breakage in right side on ALPS touchpad")
introduced a minor "typo" that broke decoding of multi-touch events are
decoded on some ALPS touchpads.  For example, tapping with three-fingers
can no longer be used to emulate middle-mouse-button (the kernel doesn't
recognize this as the proper event, and doesn't report it correctly to
userspace).  This affects touchpads that use SS4 "plus" protocol
variant, like those found on Dell E7270 & E7470 laptops (tested on
E7270).

First, probably the code in alps_decode_ss4_v2() for case
SS4_PACKET_ID_MULTI used inconsistent indices to "f->mt[]". You can see
0 & 1 are used for the "if" part but 2 & 3 are used for the "else" part.

Second, in the previous patch, new macros were introduced to decode X
coordinates specific to the SS4 "plus" variant, but the macro to
define the maximum X value wasn't changed accordingly. The macros to
decode X values for "plus" variant are effectively shifted right by 1
bit, but the max wasn't shifted too. This causes the driver to
incorrectly handle "no data" cases, which also interfered with how
multi-touch was handled.

Fixes: 4a646580 ("Input: ALPS - fix two-finger scroll breakage...")
Signed-off-by: Nir Perry <nirperry@gmail.com>
Reviewed-by: Masaki Ota <masaki.ota@jp.alps.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>