ieee1394: sbp2: optimize DMA direction of s/g tables

Unlike the name suggests, "cmd->scatter_gather_element" holds only the
s/g table, not the actual s/g elements.  Since the table is only read
but never written by the device, DMA_BIDIRECTIONAL can be replaced by
DMA_TO_DEVICE which may be cheaper on some architectures.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>