Input: ad7879 - split bus logic out

The ad7879 driver is using the old bus method of only supporting one
at a time (I2C or SPI). So refactor it like the other input drivers
that support multiple busses simultaneously.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>