mlxsw: spectrum_router: Prepare work context for possible bulking

For XMDR register it is possible to carry multiple FIB entry
operations in a single write. However the FW does not restrict mixing
the types of operations, make the code easier and indicate the bulking
is ok only in case the bulk contains FIB operations of the same family
and event.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>