thermal: generic-adc: silence info message for IIO_TEMP channels

Since commit d36e2fa0 ("thermal: generic-adc: make lookup table
optional") "generic-adc-thermal" can be used with an IIO_TEMP channel.
In this case the following message is logged at probe time:
  no lookup table, assuming DAC channel returns milliCelcius

Silence this info message if the channel type is known to be in
milli celsius. Keep this message when the channel type is unknown or not
of type temperature.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200107232044.889075-3-martin.blumenstingl@googlemail.com