Bluetooth: controller: Add missing NULL assign to df_cfg in ll_adv_set

ll_adv_set stores poitner to direction finding TX configuration.
When ll_reset is executed the pointer was not NULL assigned.
That lead to erroneous behavior e.g. df_cfg->is_enabled was set
to TRUE even the functionality was not enabled.
DF configuration is stored in memory pool. The memory pool uses
free elements to store its internal data. On reset whole pool is
expected to be free, so ll_adv_set->df_cfg may not point to any
element allocated from memory pool.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>