staging: iio: tsl2x7x: use auto increment I2C protocol

The hardware supports 16-bit ALS and proximity readings, however the
datasheet recommends using the I2C auto increment protocol so that the
correct high and low bytes are read even if the integration cycle ends
between reading the lower and upper registers.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>