Fix continuous mode readings

[pylonsoft]

If continuous mode is activated most SEN5X readings went wrong because the already read values weren't returned but get_reading() returned an error state. By checking if update() failed because of no new values available in continuous mode (SEN54 and SEN55) we can return the stored values safely.

[oscgonfer]

Thank you so much @pylonsoft ! I add a minor addition to your check for the forcedContinuousMode.

If you want/can test this last change, I'll merge!

[oscgonfer]

Also, if working with SEN55, probably take a look at #88 too

[pylonsoft]

Your addition for the forcedContinousMode works for me. I deactivated the SEN55 sensors and set the reading interval to 20 secs (both in config and for the SEN5X temperature sensor). This way the forcedContinousMode flag was activated but the continousMode flag wasn't. The readings came in as expected.