Bluetooth: BAP: Bcast Source: Refactor bt_bap_broadcast_source_reconfig

Refactor the bt_bap_broadcast_source_reconfig function to be less
complex by splitting the verification into multiple functions.

Additionally, it also fixes a rare, but potential, issue where
some subgroups were updated before a later subgroup would fail
to be updated. Added can_merge_codec_cfg_data to verify correctness
before we attempt to update any subgroups. This new check is also
applied to bt_bap_broadcast_source_create.

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

Author: Thalley