mlxsw: reg: Fix comment about slot_index field in PMAOS register

The comment did not include the register name.
Add `pmaos` to align the comment with other comments.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>