rt2x00: Rename config_preamble() to config_erp()

Rename config_preamble() to config_erp() and cleanup argument
list by putting it all into a single structure.
This will make the function more meaningful and easier to
expand later. This second option is mostly intended to make
the patch "mac80211: proper short-slot handling" from Johannes Berg
easier to apply for rt2x00.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>