ethdev: group constant definitions in Doxygen

ethdev: group constant definitions in Doxygen

A lot of flags are parts of a group but are documented alone.
The Doxygen syntax @{ and @} for grouping is used
to make flags appear together and have a common description.

Some Rx/Tx offload flags and RSS definitions are not grouped
because they need to be all properly documented first.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Kevin Traynor <ktraynor@redhat.com>