Refactoring of reset-reference-pressure action.

Optimization (info about mesuring reference pressure is displayed
      only once - after pushing the button)

After reset ref. pressure:
 * 0.0m high is displayed (only if sensor is running).
 * If sensor isn't running displayed high remains untouched.
 * If sensor isn't running N/A is displayed as a reference pressure value.

Signed-off-by: Janusz Kozerski <janusz.kozerski@gmail.com>