SML: Fix compile error when median-filter is disabled

[rPraml]

Description:

Fixes a compile error, when compiled with NO_USE_SML_MEDIAN_FILTER

In this case, sml_mf would be undefined.

Related issue (if applicable): fixes #

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.6
• The code change is tested and works with Tasmota core ESP32 V.3.0.0
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass