platform/mellanox: Add null pointer checks for devm_kasprintf()

devm_kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure.

Compile-tested only.

Fixes: 1a218d31 ("platform/mellanox: mlxbf-pmc: Add Mellanox BlueField PMC driver")
Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Suggested-by: Vadim Pasternak <vadimp@nvidia.com>
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Reviewed-by: Vadim Pasternak <vadimp@nvidia.com>
Link: https://lore.kernel.org/r/20231201055447.2356001-1-chentao@kylinos.cn


[ij: split the change into two]
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>