Bluetooth: Add PA_LINK to distinguish BIG sync and PA sync connections

Currently, BIS_LINK is used for both BIG sync and PA sync connections,
which makes it impossible to distinguish them when searching for a PA
sync connection.

Adding PA_LINK will make the distinction clearer and simplify future
extensions for PA-related features.

Signed-off-by: Yang Li <yang.li@amlogic.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>