iio: max44000: correct value in illuminance_integration_time_available

According to the datasheet, the shortest available integration time for
ALS ADC conversion is 1.5625ms but illuminance_integration_time_available
sysfs file shows wrong value.

Cc: Crestez Dan Leonard <leonard.crestez@intel.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Fixes: d5d8f49b ("max44000: Expose ambient sensor scaling")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>