* Add 11n HT40 support - i needed to send both HTINFO and HTCAP fields

* Add 11n HT40 support - i needed to send both HTINFO and HTCAP fields
  in the join message so the firmware would pick it up.

* Strip out the direct hardware fiddling for 40MHz mode - the firmware
  we're using doesn't require it (the rtl8712su firmware does; it
  is less 'fullmac' than what we're using.)

* Fix the mbuf handling during errors - rsu_tx shouldn't free mbufs;
  it's up to the caller to do so.  This brings it in line with
  what other drivers do or should be doing.

Tested:

* RTL8712, HT40 channel, STA mode (during this commit)