iio: ltr501: ltr501_read_ps(): add missing endianness conversion

iio: ltr501: ltr501_read_ps(): add missing endianness conversion

The PS ADC Channel data is spread over 2 registers in little-endian
form. This patch adds the missing endianness conversion.

Fixes: 2690be905123 ("iio: Add Lite-On ltr501 ambient light / proximity sensor driver")
Signed-off-by: Oliver Lang <Oliver.Lang@gossenmetrawatt.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Tested-by: Nikita Travkin <nikita@trvn.ru> # ltr559
Link: https://lore.kernel.org/r/20210610134619.2101372-4-mkl@pengutronix.de
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>