staging: iio: adis16203: Move header file content to source file.

The contents of the header file are used only by this single source
file. Moved content into adis16203_core.c file and removed adis16203.h
file. Arranged #include files in alphabetical order.

Signed-off-by: Varsha Rao <rvarsha016@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>