thermal/drivers/tegra: Remove unneeded lock when setting a trip point

The function tegra_tsensor_enable_hw_channel() takes the thermal zone
lock to prevent "a potential" race with a call to set_trips()
callback.

The driver must not play with the thermal framework core code
internals.

The tegra_tsensor_enable_hw_channel() is called by:

 - the suspend / resume callbacks
 - the probe function after the thermal zones are registered

The thermal zone lock taken in this function is supposed to protect
from a call to the set_trips() callback which writes in the same
register.

The potential race is when suspend / resume are called at the same
time as set_trips. This one is called only in
thermal_zone_device_update().

 - At suspend time, the 'in_suspend' is set, thus the
   thermal_zone_device_update() bails out immediately and set_trips is
   not called during this moment.

 - At resume time, the thermal zone is updated at PM_POST_SUSPEND,
   thus the driver has already set the TH2 temperature.

 - At probe time, we register the thermal zone and then we set the
   TH2. The only scenario I can see so far is the interrupt fires, the
   thermal_zone_update() is called exactly at the moment
   tegra_tsensor_enable_hw_channel() a few lines after registering it.

Enable the channels before setting up the interrupt. We close the
potential race window without using the thermal zone's lock.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Suggested-by: Thierry Reding <thierry.reding@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>