Add preliminary support for the Ralink RT5390 and RT5392 chipsets.
Committed over the D-Link DWA-525 rev A2 on amd64 with WPA.

Rename definition of IEEE80211_FC1_WEP to IEEE80211_FC1_PROTECTED.

The origin of WEP comes from IEEE Std 802.11-1997 where it defines
whether the frame body of MAC frame has been encrypted using WEP
algorithm or not.
IEEE Std. 802.11-2007 changes WEP to Protected Frame, indicates
whether the frame is protected by a cryptographic encapsulation
algorithm.

Reviewed by:	adrian, rpaulo

* Add 802.11n 2ghz and 5ghz tables, including legacy rates and up to
  MCS23 rates (3x3.)

* Populate the rate code -> rate index lookup table with MCS _and_
  normal rates, but _not_ the basic rate flag.  Since the basic rate flag
  is the same as the MCS flag, we can only use one.

* Introduce some accessor inlines that do PLCP and rate table lookup/access
  and enforce that it doesn't set the basic rate bit.  They're not
  designed for MCS rates, so it will panic.

* Start converting drivers that use the rate table stuff to use the
  accessor inlines and strip the basic flag.

* Teach AMRR about basic 11n - it's still as crap for MCS as it is
  being used by iwn, so it's not a step _backwardS_.

* Convert iwn over to accept 11n MCS rates rather than 'translate' legacy
  to MCS rates.  It doesn't use a lookup table any longer; instead it's a
  function which takes the current node (for HT parameters) and the
  rate code, and returns the hardware PLCP code to use.

Tested:

* ath - it's a no-op, and it works that way
* iwn - both 11n and non-11n

malloc(9) flags in sys/dev.

Thanks to ray@, Sevan and Sergey Dyatko for feedback and testing!

Obtained from:	OpenBSD
MFC after:	3 weeks