platform/mellanox: mlxbf-pmc: Cleanup signed/unsigned mix-up

Use unsigned integer types for register values and array indices.
Use %u instead of %d accordingly.
Signed-off-by: Shravan Kumar Ramani <shravankr@nvidia.com>
Link: https://lore.kernel.org/r/d8548c70339a29258a906b2b518e5c48f669795c.1707808180.git.shravankr@nvidia.comReviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>