iio:light:apds9960 Fix sparse endian warnings.

This patch is a tidy up of warnings from the autobuilder.

>> drivers/iio/light/apds9960.c:495:32: sparse: cast to restricted __le16
   drivers/iio/light/apds9960.c:635:24: sparse: cast to restricted __le16
>> drivers/iio/light/apds9960.c:672:21: sparse: incorrect type in assignment (different base types)
   drivers/iio/light/apds9960.c:672:21:    expected unsigned short [unsigned] [usertype] buf
   drivers/iio/light/apds9960.c:672:21:    got restricted __le16 [usertype] <noident>

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: mranostay@gmail.com
Signed-off-by: Jonathan Cameron <jic23@kernel.org>