Allow to not change/set SR params

[Jaaaky]

Hello,
using Ardudeck alpha-29
For message rates your should allow not setting rates at all "-1", like this in MissionPlanner

Or mavproxy:

set streamrate -1

On telemetry it's very important, you should not change carefully set/calculated rates otherwise very bad degraded performance is expected.

Also normally you should simply ask for message rates not setting SR* params. Setting SR param is permanent and undesirable. while simply using MAV_DATA_STREAM request should be transient.