mlxsw: reg: Increase size of MPAR register

In new firmware versions this register is extended with a sampling rate
for Spectrum-2 and future ASICs.

Increase the size of the register to ensure the field is initialized to
0 which means every packet is mirrored.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>