dpaa2-eth: fix ethtool statistics

dpaa2-eth: fix ethtool statistics

Unfortunately, with the blamed commit I also added a side effect in the
ethtool stats shown. Because I added two more fields in the per channel
structure without verifying if its size is used in any way, part of the
ethtool statistics were off by 2.
Fix this by not looking up the size of the structure but instead on a
fixed value kept in a macro.

Fixes: eb2caa7be715 ("net: dpaa2: add adaptive interrupt coalescing")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Link: https://lore.kernel.org/r/20211215105831.290070-1-ioana.ciornei@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>