iio: kfifo: Set update_needed to false only if a buffer was allocated

Check whether the allocation of a new kfifo buffer failed or not before
setting the update_needed flag to false. This will make
iio_request_update_kfifo() try to allocate a new buffer the next time a
buffer update is requested.

Signed-off-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>