mlxsw: reg: Document possible MCIA status values

Will be used to emit meaningful messages to user space via extack in a
subsequent patch.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>