iio: core: add missing IIO_MOD_H2/ETHANOL string identifiers

Add missing strings to iio_modifier_names[] for proper modification
of channels.

Fixes: b170f7d4 (iio: Add modifiers for ethanol and H2 gases)
Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>