mac80211: fix rates setup on IBSS merge

when an IBSS merge happened, the supported rates for the newly added station
were left empty, causing the rate control module to be initialized with only
the basic rates.

also the section of the ibss code which deals with updating supported rates for
an already existing station fails to inform the rate control module about the
new rates. as i don't know how to fix this (minstrel does not have an update
function), i have just added a comment for now.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>