iio: buffer: Drop get_length callback

We already do have the length field in the struct iio_buffer which is
expected to be in sync with the current size of the buffer. And currently
all implementations of the get_length callback either return this field or a
constant number.

This patch removes the get_length callback and replaces all occurrences in
the IIO core with directly accessing the length field of the buffer.

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

diff --cc drivers/iio/industrialio-buffer.c
Simple merge

diff --cc drivers/iio/kfifo_buf.c
Simple merge

diff --cc drivers/staging/iio/Documentation/ring.txt
Simple merge

diff --cc drivers/staging/iio/accel/sca3000_ring.c
Simple merge

diff --cc include/linux/iio/buffer.h
Simple merge