mlxsw: spectrum_switchdev: Use extack in bridge port flag validation

Propagate extack to mlxsw_sp_port_attr_br_pre_flags_set() in order to
communicate error messages related to bridge port flag validation.

Example:

 # bridge link set dev swp1 locked on
 Error: mlxsw_spectrum: Unsupported bridge port flag.

More error messages will be added in subsequent patches.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>