ltr502als.c: Modify the way to change the config register

ltr502als.c: Modify the way to change the config register

BZ: 26351

When change the device mode, we need to set the CONFIG REG to idle mode
first to disable interrupt. Otherwise, some interrupts may be triggered
abnormally during the mode switch.

Change-Id: I67792fce52fc1377aed9b5316b84a122451c7fde
Signed-off-by: Leo Yan <leo.yan@intel.com>
Reviewed-on: http://android.intel.com:8080/38779
Reviewed-by: Liu, Hong <hong.liu@intel.com>
Reviewed-by: Du, Alek <alek.du@intel.com>
Reviewed-by: Koskinen, Ilkka <ilkka.koskinen@intel.com>
Reviewed-by: Monroy, German <german.monroy@intel.com>
Tested-by: Wang, Zhifeng <zhifeng.wang@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>