mlxsw: reg: Use correct offset in field definiton

The rx_lane, tx_lane and module fields in the PMLP register don't have
an additional offset besides the base one (0x04), so set it to 0x00.

Fixes: 4ec14b76 ("mlxsw: Add interface to access registers and process events")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>