Bluetooth: Audio: Fix issue with registering AICS callbacks

When registering AICS callbacks via bt_vcs_client_cb_register
or bt_micp_mic_ctlr_cb_register it could fail if the application
did not reset the aics_cb.discover as that is set internally.

This commit adds a simple check to also allow the
aics_cb.discover to be the callback they expect/want it to be.

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