mlxsw: spectrum_router: Clarify a comment

"Reserved for X" usually means that only X is supposed to use a given
object. Here, it is used in the sense that X should consider the object
"reserved", as in "restricted".

Replace the comment simply by "X", with the implication that that's where
the field is used.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>