Bluetooth: controller: Fix error when max ant. pattern len not defined

Fix compilation error if CONFIG_BT_CTLR_DF is not defined.
If DF was not enabled, there was missing CONFIG_BT_CTLR_DF_MAX_-
ANT_SW_PATTERN_LEN and undeclared symbol was found.

Split ll_df.h into two parts:
- ll_df.h that is generally includeable and allows to successfully
  compile sources when DF is not enabled.
- ll_df_internal.h to be used in soucres that are compiled when
  DF is enabled.

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