staging: fsl-dpaa2/eth: Fix SGT allocation

We mistakenly allocate space for too many entries in the
scatter-gather table of multi buffer egress frames.

While it doesn't have a negative impact from a functional
point of view, it wastes resources so fix it.

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