mlxsw: Define Tx header fields in txheader.h

mlxsw: Define Tx header fields in txheader.h

The next patch will move Tx header constructing to pci.c. As preparation,
move the definitions of Tx header fields from spectrum.c to txheader.h,
so pci.c will include this header and can access the fields.

Remove 'etclass' which is not used.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://patch.msgid.link/2250b5cb3998ab4850fc8251c3a0f5926d32e194.1737044384.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>