mlxsw: reg: Increase 'port_num' field in PMTDB register

'port_num' field is used to indicate the local port value which can be
assigned to a module.

Increase the field from 8 bits to 10 bits in order to support more than
255 ports.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>