mlxsw: reg: Use correct PBMC register length

The last field of the PBMC register is at offset 0x64 and its size is
0x8, so the correct register's length is 0x6C bytes.

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>