dpaa2-eth: fixup kdoc warnings

Running kernel-doc over the dpaa2-eth driver generates a bunch of
warnings. Fix them up by removing code comments for macros which are
self-explanatory, respecting the kdoc format for macro documentation and
other small changes like describing the expected return values of
functions.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>