Use the new dscp_t type to replace the tos field of struct
mlxsw_sp_fib4_entry. This ensures ECN bits are ignored and makes it
compatible with the dscp fields of fib_entry_notifier_info and
fib_rt_info.

This also allows sparse to flag potential incorrect uses of DSCP and
ECN bits.
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>