juno/adc: Trust the HAL in mod_sensor_driver_api

This driver implements an API defined by the HAL, so
we can be sure that the parameters passed to us do
not need to be sanitized.

Change-Id: I74aa1ab9a46fa7d8757c87fd85ef5965f1582151
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>