thermal: Get rid of CONFIG_THERMAL_WRITABLE_TRIPS

thermal: Get rid of CONFIG_THERMAL_WRITABLE_TRIPS

The only difference made by CONFIG_THERMAL_WRITABLE_TRIPS is whether or
not the writable trips mask passed during thermal zone registration
will take any effect, but whoever passes a non-zero writable trips mask
to thermal_zone_device_register_with_trips() can be forgiven thinking
that it will always work.

Moreover, some thermal drivers expect user space to set trip temperature
values, so they select CONFIG_THERMAL_WRITABLE_TRIPS, possibly overriding
a manual choice to unset it and going against the design purportedly
allowing system integrators to decide on the writability of trip points
for the given kernel build.  It is also set in one platform's defconfig.

Forthermore, CONFIG_THERMAL_WRITABLE_TRIPS only affects trip temperature,
because trip hysteresis is writable as long as the thermal zone provides
a callback to update it, regardless of the CONFIG_THERMAL_WRITABLE_TRIPS
value.

The above means that the symbol in question is used inconsistently and
its purpose is at least moot, so remove it and always take the writable
trip mask passed to thermal_zone_device_register_with_trips() into
account.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>