Input: ad7879-i2c - use swapped variant of i2c_smbus_read_word_data

This variant was introduced in

i2c: boilerplate function for byte swapped smbus_write/read_word_data

This also has the side effect of ensuring any errors from the i2c
read and no longer mangled.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>