Convert to use bus-agnostic DMA routines to make it usable on AHB bus as well.

Changes-licensed-under: ISC
Signed-off-by: NGabor Juhos <juhosg@openwrt.org>
Signed-off-by: NImre Kaloz <kaloz@openwrt.org>
Tested-by: NPavel Roskin <proski@gnu.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>