Basic rates setting is done with hardcoded register write with
fixed settings.

This patch introduces a new function that makes it possible to
configure basic rates and it add a check for mac80211
BSS_CHANGED_BASIC_RATES flag in order to eventually invoke that
function when needed.
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>