iio: qcom-rradc: Disable continuous mode when read fails

Currently, when a channel is read in continuous mode and the read
operation fails, RR_ADC would be left enabled in continuous mode.
Disable the continuous mode in such cases so that the other read
operations which doesn't need continuous mode can go through.

Change-Id: I2bf257bd535e1e4a30e18b6257e584a5be96b69d
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>