staging:iio: Rename IIO_EVENT_CODE_EXTRACT_NUM to IIO_EVENT_CODE_EXTRACT_CHAN

We name this field "chan" throughout IIO with the exception of this one macro.
Rename it to be more consistent.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>