hwmon: (lm90) Fix handling of hysteresis value

There are several problems in the way the hysteresis value is handled
by the lm90 driver:

* In show_temphyst(), specific handling of the MAX6646 is missing, so
  the hysteresis is reported incorrectly if the critical temperature
  is over 127 degrees C.
* In set_temphyst(), the new hysteresis register value is written to
  the chip but data->temp_hyst isn't updated accordingly, so there is
  a short period of time (up to 2 seconds) where the old hystereris
  value will be returned while the new one is already active.
* In set_temphyst(), the critical temperature which is used as a base
  to compute the value of the hysteresis register lacks
  device-specific handling. As a result, the value of the hysteresis
  register might be incorrect for the ADT7461 and MAX6646 chips.

Fix these 3 bugs.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Cc: Nate Case <ncase@xes-inc.com>