libertas: implement new scanning logic

This changes the code that is used for scanning and makes it hopefully
easier to understand:

* move function into logical blocks
* create a bunch of lbs_scan_add_XXXX_tlv() functions, that
  help to create the TLV parameter of CMD_802_11_SCAN
* all of them are now called from the much simpler lbs_do_scan()
* no **puserscancfg double-pointers :-)

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>