i2s audio, some small improvements

[Staars]

Description:

1. Prevents unworkable default mode PDM for microphone on SOC's that do not support PDM mode (e.g. ESP-S2).
2. Prevents crash when attempting to record in stereo mode, which does not really make sense on a mono microphone, but at least it is possible now.
3. Less strict memory management for the MP3 encoder, which does not need preallocation anymore in the driver settings. Now late allocation is supported, but preallocation is still possible and it can be still useful, if free (large blocks of) heap is not guaranteed later in the running system.

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