staging: brcm80211: cleaned up softmac DMA layer

Code cleanup. Since this driver only needs to support 64 bit DMA hardware,
an unnecessary layer of abstraction could be removed. Also DMA functions that
were not called have been removed.

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