For edma, we should use DMA_BIDIRECTIONAL, or else use
DMA_FROM_DEVICE.

This is found to address "BUG at arch/x86/mm/physaddr.c:5"
as described here:

http://lkml.org/lkml/2010/7/14/21Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>