iio: kfifo_buf: Implement data_available() callback

This patch implements the data_available() callback for the kfifo buffer instead
of using the stufftoread flag. The kfifo used by the buffer already knows
whether it is empty or not based on the position of its read and write pointer.
Using this makes it a lot easier to tell whether data is available or not and it
is not necessary to take special measures to ensure that no race conditions
between reading and writing from the buffer occur.

Note, that we still have to take the buffers lock to protect against concurrent
resizeing of the kfifo.

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