libertas: rate adaptation configuration via iwconfig.

Implemented rate adaptation support via 'iwconfig rate' API.  It is now
possible to specify a bit-rate value and append 'auto'.  That will configure
rate adaptation to use all bit-rates equal or lower than than selected value.

Made lbs_cmd_802_11_rate_adapt_rateset a direct command.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>