This patch makes the w83781d driver preserve fan_min settings when the
user changes fan_divs. This isn't done "by default" because the actual
fan min value (in RPMs) depends on both the fan_min register and the
fan_div register. Only two drivers handle it properly at the moment as
far as I know (lm78 and asb100). Several other drivers would need to be
fixed the same way, but well, once at a time ;)

Tested on my AS99127F rev.1.

Credits go to Philip Pokorny, since I think I remember he is the one who
introduced the method in the lm78 driver in the first place.

This tends to increase the size of the three set_store_regs_fan_div
functions, and I am considering refactoring them at some point. Later
though.