Reorganize configuration handling by creating a extra
structure which contains precalculated values based
on the mac80211 values which are usefull for all
individual drivers.

This also fixes the preamble configuration problem,
up untill now preamble was never configured since
by default the rate->val value was used when changing
the mode.
Now rate->val will only be used to set the basic rate mask.
The preamble configuration will now be done correctly
through the erp_ie_changed callback function.
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>