staging: fsl-dpaa2/eth: Fix DMA mapping direction

We are using DMA_FROM_DEVICE when mapping RX frame buffers,
but DMA_BIDIRECTIONAL for unmap. Fix the direction for DMA
unmapping operation.

Fixes: 87eb55e4 ("staging: fsl-dpaa2/eth: Fix potential endless loop")

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>