platform/mellanox: mlxbf-pmc: fix sscanf() error checking

The sscanf() function never returns negatives.  It returns the number of
items successfully read.

Fixes: 1a218d31 ("platform/mellanox: mlxbf-pmc: Add Mellanox BlueField PMC driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/4ccdfd28-099b-40bf-8d77-ad4ea2e76b93@kili.mountain


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>