Bluetooth: Audio: Avoid error log for bidirectional CIS

In the case that the CIS has been setup as bidirectional, and
only one of the directions have an ASE configured yet,
we should only care about valid ISO packets when doing this
check. The reason is that some controllers send HCI ISO data
packets to the host, even if no SDU was sent on the remote
side. This basically means that empty PDUs are sent to the
host as HCI ISO data packets, which we should just ignore.

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