Bluetooth: MICP: Add mic_ctlr_get_by_conn

Add function to get a MICP microphone controller
instance from a connection pointer. This is effectively
the reverse of bt_micp_mic_ctlr_conn_get, and works similar to
bt_vcp_vol_ctlr_get_by_conn.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>