mac802154: move header parse functions to rx.c

This patch moves the frame parsing part inside the rx function. It's
part of receive handling until netif_rx_ni is called.
mac802154: rename sdata slaves and slaves_mtx

This patch namens the slaves attribute in sdata to interfaces and
slaves_mtx to iflist_mtx. This patch is part to get the wireless feeling
into the 802.15.4 implementation.
mac802154: introduce IEEE802154_DEV_TO_SUB_IF

This function adds a wrapper to call netdev_priv to getting the sdata
attribute. There exists a similar function in the wirless stack.
mac802154: move slave open/close functions to wpan

This function is never used outside of wpan.c and can be static there.
This patch moves these functions into the wpan.c file.
mac802154: rename wpan.c to iface.c

The wpan.c file contains interface handling functions now. This file is
similar like the mac80211 iface.c. We rename this file to iface.c to get
the wireless feeling into the 802.15.4 implementation.
mac802154: rx: use netif_receive_skb

To use netif_receive_skb or netif_rx_ni depends on context. We have a
tasklet which is the same like mac80211 and we should use the same
net rx delivery function netif_receive_skb.
mac802154: introduce internal driver-ops header

We should not call the driver ops callbacks directly. This patch
introduce a driver-ops header file to calling these wrappers. It's like
the mac80211 driver-ops header.
mac802154: remove useless -EBUSY if sdata running

The slave_open function is only called by ndo_open callback. This
callback is already protected to do a ip link set wpan0 up twice.
This patch removing also a unnecessary for each loop to find the right
entry which is already known.
mac802154: rework open count

Some cleanup to avoid pre- and post-calculation with the open_count
variable.
mac802154: remove ieee802154_addr from driver_ops

This function is never used by any driver. There exist also no comment
for this. In my opinion it was a try to implement hardware address
filter but we have already such driver ops for this functionallity.
mac802154: rework sdata state change to running

This patch reworks the handling for setting the state like mac80211. We
use bit's instead a bool variable. The mutex is not needed because it's
locked by rtnl lock.