staging:iio:ad7606: Move buffer code to main source file

Currently the ad7606 buffer handling code resides in its own source file.
But this file contains only 4 small functions of which half are just
wrappers around other functions. Buffer support is also always enabled for
this driver, so move them over to the main source file. This reduces the
amount of boilerplate code.

Also rename the main function from ad7606_core.c to ad7606.c since there is
only a single file now.

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