Add ethanol and H2 gas modifiers: * IIO_MOD_ETHANOL * IIO_MOD_H2 Signed-off-by: Andreas Brauchli <andreas.brauchli@sensirion.com> Acked-by: Matt Ranostay <matt.ranostay@konsulko.com> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>