revert previous commit (rev. 254082 / split 898bcc3c)
set max Rx size and ampdu density

When an 11n capable station try to associate with an AP, the station does
not include maximum Rx size and ampdu density set by the driver into an
association request packet. So that, the AP might generate ampdu packets
bigger than the station can handle.

They cannot apply this simple patch, so I did it by myself.
http://lists.freebsd.org/pipermail/freebsd-wireless/2013-February/002894.html