mhz19b_sample_fetch() from mhz19b sensor driver didn't support
SENSOR_CHAN_ALL chan parameter value, so sensor_sample_fetch() didn't
work, always returning -ENOTSUP (sensor_sample_fetch_chan() worked, if
called with SENSOR_CHAN_CO2).
This change enables mhz19b sensor to work both with
sensor_sample_fetch() and with sensor_sample_fetch_chan() with
SENSOR_CHAN_CO2.
Signed-off-by: Konstantin Mochalov <incredible.angst@gmail.com>