thermal: generic-adc: silence "no lookup table" on deferred probe

A "generic-adc-thermal" without "temperature-lookup-table" is perfectly
valid since commit d36e2fa0 ("thermal: generic-adc: make lookup
table optional"). On deferred probe the message "no lookup table,
assuming DAC channel returns milliCelcius" is still logged.
Prevent this message on deferred probe of the IIO channel by first
looking up the IIO channel.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200107232044.889075-2-martin.blumenstingl@googlemail.com