Bluetooth: BAP: SD: Add check for mixing NO_PREF with specific BIS

Based on a dicussion on the Bluetooth SIG GAWG reflector, it is
not allowed for a broadcast assistant to request specific BIS
indexes as well as BT_BAP_BIS_SYNC_NO_PREF in the same
request.

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