Bluetooth: Host: Fix recv_enabled field for PAST

When syncing to a PA using PAST then the sync_info.recv_enabled
was always just set to true, regardless of what mode was set
during the subscribe parameters.

The mode(s) are now stored in an array (with the default value
as well) so that we can retrieve that information when the PA
has synced via PAST.

It was considered to put the `mode` value into the `bt_conn`
struct, but that would require an API change as the `bt_conn`
parameter for the subcribe function uses `const`.

This commit also modifies the guard for PAST to be the more
correct value CONFIG_BT_PER_ADV_SYNC_TRANSFER_RECEIVER instead
of just CONFIG_BT_CONN.

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