wacom: Fixes for stylus pressure values for Thinkpad Yoga

[Carl Worth]

This series of patches fixes the pressure values reported for the
wacom tablet built-in to a Lenovo ThinkPad Yoga laptop. Prior to this
patch series, if I slowly increased stylus pressure, (expecting a
gradual increase of values from 0 to 1023), I instead received values
that increased slowly to 255, then reset to 0 and increased slowly
again, etc.

The buggy arithmetic that is updated here appears to exist in
identical forms for other drivers. I did not update any code that I
was not able to test directly. But it looks like wacom_pl_irq and
wacom_dtu_irq potentially have similar bugs, (depending on the actual
pressure_max values encountered in practice).

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