dma: edma: fix slave config dependency on direction

The edma_slave_config() implementation depends on the
direction field such that it will not properly configure
a slave channel when called without direction set.

This fixes the implementation so that the slave config
is copied as is and prep_slave_sg() handles the
direction dependent handling. spi-omap2-mcspi and
omap_hsmmc both expose this bug as they configure the
slave channel config from a common path with an unconfigured
direction field.

Signed-off-by: Matt Porter <mporter@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>