Staging: iio: fix ring buffer build

max1363 uses both the iio hardware ring buffer and software
ring buffer interfaces, but its Makefile and Kconfig do not
reflect that usage, so its build breaks.  Add a new Kconfig
symbol to reflect that usage and change max1363.h & Makefile
to use the new Kconfig symbol.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>