drivers: sensor: ccs811: accommodate old firmware

The firmware version on the CCS811 on nrf52_pca20020 is generally
version 1.1, which does not set the data ready bit in the status
register.  Assume that a non-zero CO2 measurement on that version is
fresh for the purposes of determining the fetch status.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>