als: fix measurement with very low ambient light

When covering the sensor, it gives a measurement of 47418 due to
arithmetic operations on unsigned types.

Change-Id: Icc110fc4aa11286e7a072bc3ac01dadfe45617c2
Tracked-On: https://jira01.devtools.intel.com/browse/AW-4487


Signed-off-by: Hubert CHAUMETTE <hubertx.chaumette@intel.com>
Reviewed-on: https://android.intel.com/573584


Reviewed-by: jenkins_ndg <jenkins_ndg@intel.com>
Reviewed-by: Ghaddab, RiadhX <riadhx.ghaddab@intel.com>
Reviewed-by: Dubray, SimonX <simonx.dubray@intel.com>
Reviewed-by: Hu, Bingquan <bingquan.hu@intel.com>
Reviewed-by: Tasayco Loarte, VictorX <victorx.tasayco.loarte@intel.com>