drivers: sensor: ccs811: bypass APP_START when not in boot mode

The CCS811 can be in either boot mode (after powerup) or application
mode, and these modes have distinct register maps that share only some
content.  The register written to switch from boot mode to application
mode is not available in application mode, so don't write to it in that
case.

Also respect the required timeout between APP_START and next I2C
operation.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>