Bluetooth: ISO: Extract and store information about RX and TX

Instead of relying only on the `tx` and `rx` qos pointers,
we extract further information from the
bt_hci_evt_le_cis_established event to properly determine if we
can actually send or receive data.

This is useful to help determine which data paths to setup,
and whether to reject requests to send.

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