iio: buffer: kfifo_buf: kernel-doc, typo in function name.

Should have been _kfifo_ and was _fifo_

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20210314164655.408461-9-jic23@kernel.org