mlxsw: Make PMAOS pack function more generic

The PMAOS register has enable bits (e.g., PMAOS.ee) that allow changing
only a subset of the fields, which is exactly what subsequent patches
will need to do. Instead of passing multiple arguments to its pack
function, only pass the module index and let the rest be set by the
different callers.

No functional changes intended.

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