staging: fsl-dpaa2/eth: Minor cleanup in dpaa2_eth_set_hash

We already have a variable for the DMA mapping device,
so use that directly.

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