mlxsw: spectrum_fid: Extract SFMR packing into a helper

Both mlxsw_sp_fid_op() and mlxsw_sp_fid_edit_op() pack the core of SFMR the
same way. Extract the common code into a helper and call that. Extract out
of that a wrapper that just calls mlxsw_reg_sfmr_pack(), because it will
be useful for the dummy family later on.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/31f32b4d767183f6cb197148d0792feab2efadba.1700503644.git.petrm@nvidia.com


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