drivers: sensor: vl53l0x: Use proper variable type for return code

VL53L0X_PerformSingleRangingMeasurement() returns a signed 8-bit
integer, not an unsigned 8-bit integer, making the "< 0" comparison
worthless.

Coverity-CID: 182593
Coverity-CID: 182597
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>