staging: brcm80211: changed comment on rx buffer len in wlc_ampdu.c

Code cleanup. Code in wlc_ampdu.c is waiting for Mac80211 functionality to be released.
Added this information to a code comment.

Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>