mlxsw: reg: Extend PMLP tx/rx lane value size to 4 bits

The tx/rx lane fields got extended to 4 bits, update the reg field
description accordingly.

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