iio:ad5446: get rid of private header file

Most of the defines in there were not even used, and the structs left are
private to the .c file. Makes the driver more in line with most of the
kernel drivers.

Signed-off-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>