therm_adt746x: Always clear hardware bit which inverts fan speed range.

This bit would get enabled sometimes (probably after suspend/resume), so the
fan would run at full speed below the temperature thresholds, but slow down and
eventually stop if temperatures rose above the thresholds... not exactly what
you want.

Signed-off-by: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>