mlxsw: Do not store Tx header length as driver parameter

Tx header handling was moved to PCI code, as there is no several drivers
which configure Tx header differently. Tx header length is stored as driver
parameter, this is not really necessary as it always stores the same value.
Remove this field and use the macro MLXSW_TXHDR_LEN explicitly.

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/1fb7b3f007de4d311e559c8a954b673d0895d5e9.1737044384.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>