Bluetooth: BAP: Update log for unicast iso recv without endpoint

Since we always set up the ISO data path for endpoints in both
directions (due to limitations in the ISO API), we can actually
receive valid (empty) SDUs on a CIS in a direction that has not
been configured.

This is not the ideal solution, but prevents unncessary LOG_ERR.
The ideal solution is to modify the ISO API to provide the
flexibility that BAP needs w.r.t. ISO data paths, but that is
a larger change for later.

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