drivers: sensor: ntc-thermistor-generic: add missing mutex

The ntc-thermistor-generic is not thread safe when calling
sensor_channel_get() due to the mutex not being used in
ntc_thermistor_channel_get() when the sampled data is accessed.
Add the thermistor_data mutex around the data access to fix.

Signed-off-by: Tom Deconinck <t.deconinck@gmail.com>