iio: core: Simplify the registration of kfifo buffers

iio: core: Simplify the registration of kfifo buffers

Among all the users of the kfifo buffers, no one uses the
INDIO_BUFFER_HARDWARE mode. So let's take this as a general rule and
simplify a little bit the internals - overall the documentation - by
eliminating unused specific cases. Use the INDIO_BUFFER_SOFTWARE mode by
default with kfifo buffers, which will basically mimic what all the "non
direct" modes do.

Cc: Benson Leung <bleung@chromium.org>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: Jyoti Bhayana <jbhayana@google.com>
Cc: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Cc: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220207143840.707510-13-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>