net/ieee802154: Simplify data FS settings function signature

There is no need of interface anymore.
After previous changes, it appears that net_if parameter is not used so
we can remove it.

Change-Id: Id3570f50865696818a9be2280172e2e25fc537f7
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>