iio: adc: tsc2046: fix a warning message in tsc2046_adc_update_scan_mode()

These variables are unsigned so the condition can't be less than zero
and the warning message will never be printed.

Fixes: 9374e8f5 ("iio: adc: add ADC driver for the TI TSC2046 controller")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YJ+ZuO43TnguY5vq@mwanda


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>