Input: synaptics - set min/max for finger width

Reporting this will allow GUI config apps to correctly scale
width sensitive config values (such as palm detect) to correct
range.  Current user apps are detecting kernels min/max=0/0 and
making an assumption that it means 0/16 or 0/15.

Synaptics touchpad interface guides show 4/15 are correct values
but driver forces to 0 when no fingers on touchpad.

Signed-off-by: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>