mlxsw: Adjust some MFDE fields shift and size to fw implementation

MFDE.irisc_id and MFDE.event_id were adjusted according to what is
actually implemented in firmware.

Adjust the shift and size of these fields in mlxsw as well.

Note that the displacement of the first field is not a regression.
It was always incorrect and therefore reported "0".

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