iio:kfifo_buf improve error handling in read_first_n.

These two elements were originally in the patch
iio:kfifo_buf  Take advantage of the fixed record size used in IIO
but Lars-Peter Clausen pointed out they should not have been
so here they are.

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