staging: brcm80211: added clarification on softmac dma alignment

Descriptor ring can only start at 8KB alignment, this requirement
is dictated by DMA hardware.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>