staging/ad7606: Actually build the interface modules

The ad7606_par and ad7606_spi drivers are not built if CONFIG_AD7606=m,
because kbuild does not currently support <objname>-m syntax. Even if we
add kbuild support, ad7606 fails to link, because of duplicate
module_init definitions. Make the two drivers separate modules, as the
Kconfig help text already suggests.

Also, CONFIG_IIO_BUFFER is a dependency of CONFIG_AD7606, so there is no
need to test for it in the Makefile.

Signed-off-by: Michal Marek <mmarek@suse.com>