staging:iio:adxrs450: Move header file contents to main file

The contents of the adxrs450 header file is not used outside the main drivers
file, so just move the contents from the header file into the drivers main file.
Also rename the adxrs450 driver file from adxrs450_core.c to adxrs450.c.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>