mlxsw: reg: Add "mgpir_" prefix to MGPIR fields comments

Do the same as for other registers and have "mgpir_" prefix for the
MGPIR fields.

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