wimax/i2400m: support extended data RX protocol (no need to reallocate skbs)

Newer i2400m firmwares (>= v1.4) extend the data RX protocol so that
each packet has a 16 byte header. This header is mainly used to
implement host reordeing (which is addressed in later commits).

However, this header also allows us to overwrite it (once data has
been extracted) with an Ethernet header and deliver to the networking
stack without having to reallocate the skb (as it happened in fw <=
v1.3) to make room for it.

- control.c: indicate the device [dev_initialize()] that the driver
  wants to use the extended data RX protocol. Also involves adding the
  definition of the needed data types in include/linux/wimax/i2400m.h.

- rx.c: handle the new payload type for the extended RX data
  protocol. Prepares the skb for delivery to
  netdev.c:i2400m_net_erx().

- netdev.c: Introduce i2400m_net_erx() that adds the fake ethernet
  address to a prepared skb and delivers it to the networking
  stack.

- cleanup: in most instances in rx.c, the variable 'single' was
  renamed to 'single_last' for it better conveys its meaning.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>