Bluetooth: controller: Fix link_tx overwrite at Central CIS creation

In the case where memq_deinit in LLL flushing ends up with the free
link being the link provided by the CIS instance, and pointed to by
lll.link_tx_free, the free counter data in the link element is
overwritten during central CIS creation.

This has the effect that when starting the next CIG, there will suddenly
be 0 links available, and controller fails assertion.

By saving- and restoring the lll.link_tx before and after CIS
configuration cache copying, the free counter is intact.

Signed-off-by: Morten Priess <mtpr@oticon.com>