Bluetooth: Audio: add bt_audio_get_chan_count

Implement a function bt_audio_get_chan_count that takes an enum
bt_audio_location and returns the number of channels in that value.

This PR fixes #69617
(https://github.com/zephyrproject-rtos/zephyr/issues/69617

)

Signed-off-by: Babak Arisian <bbaa@demant.com>