ACPI / thermal: do not always return THERMAL_TREND_RAISING for active trip points

Commit 4ae46be "Thermal: Introduce thermal_zone_trip_update()"
introduced a regression causing the fan to be always on even when
the system is idle.

My original idea in that commit is that:
 - when the current temperature is above the trip point,
   keep the fan on, even if the temperature is dropping.
 - when the current temperature is below the trip point,
   turn on the fan when the temperature is raising,
   turn off the fan when the temperature is dropping.

But this is what the code actually does:
 - when the current temperature is above the trip point,
   the fan keeps on.
 - when the current temperature is below the trip point,
   the fan is always on because thermal_get_trend()
   in driver/acpi/thermal.c returns THERMAL_TREND_RAISING.
Thus the fan keeps running even if the system is idle.

Fix this in drivers/acpi/thermal.c.

[rjw: Changelog]
References: https://bugzilla.kernel.org/show_bug.cgi?id=56591
References: https://bugzilla.kernel.org/show_bug.cgi?id=56601
References: https://bugzilla.kernel.org/show_bug.cgi?id=50041#c45
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Tested-by: Matthias <morpheusxyz123@yahoo.de>
Tested-by: Ville Syrjälä <syrjala@sci.fi>
Cc: 3.7+ <stable@vger.kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>