Input: synaptics - fix handling of disabling gesture mode

Bit 2 of the mode byte has dual meaning: it can disable reporting of
gestures when touchpad works in Relative mode or normal Absolute mode,
or it can enable so called Extended W-Mode when touchpad uses enhanced
Absolute mode (W-mode). The extended W-Mode confuses our driver and
causes missing button presses on some Thinkpads (x250, T450s), so let's
make sure we do not enable it.

Also, according to the spec W mode "... bit is defined only in Absolute
mode on pads whose capExtended capability bit is set. In Relative mode and
in TouchPads without this capability, the bit is reserved and should be
left at 0.", so let's make sure we respect this requirement as well.

Reported-by: Nick Bowler <nbowler@draconx.ca>
Suggested-by: Gabor Balla <gaborwho@gmail.com>
Tested-by: Gabor Balla <gaborwho@gmail.com>
Tested-by: Nick Bowler <nbowler@draconx.ca>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>