a2dp: Fix invalid channel mode conversion

ChannelMode::STEREO was incorreclty converted back
to BTAV_A2DP_CODEC_CHANNEL_MODE_MONO (typo)

Bug: 305734815
Bug: 308686081
Test: m com.android.btservices
Change-Id: If7ee40952ce611711a30206604e96975567b3ceb