mmdevice: don't report headphones if not stereo

Fixes the following assert:

vlc: ../../src/audio_output/output.c:794: aout_OutputNew: Assertion `fmt->i_physical_channels == AOUT_CHANS_STEREO' failed.

Fixes #27371 (closed)

One other way to fix this issue would to remove the assert and replace it by a strong check.