Bluetooth: BAP: Shell: Fix bad PA sync ref for sink auto scan

When creating a broadcast sink using the auto scan feature,
the call to bt_le_per_adv_sync_create would use a local
PA sync pointer, instead of one from the per_adv_syncs
array, making it impossible to stop the PA sync afterwards.

This commit modifed the auto_scan so that it properly uses
the per_adv_syncs array, while still assigning the PA sync
in the shell broadcast sink struct.

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