tablet: reduce the pressure range by the offset

Previously, the pressure range was calculated from the axis total range. A
device with a pressure offset making the bottom 10% inaccessible would lose
10% of that range as non-accessible. Due to the implementation, this affected
the upper range of the device, so the top N percent became unaccessible. Which
may be why no-one's noticed this yet.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>