Bluetooth: BAP: Modify range of BT_BAP_BROADCAST_SNK_SUBGROUP_COUNT

The range of BT_BAP_BROADCAST_SNK_SUBGROUP_COUNT should be relative to
BT_BAP_BASS_MAX_SUBGROUPS. The broadcast sink depends on the scan
delegator, and thus it does not make sense for the broadcast sink
to support more subgroups than what the scan delegator supports.

Since this no longer relates to the number of streams, there
may be new runtime errors if applications attempt
to sync to more subgroups than the number of ISO channels
supported.

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