drivers: base: swnode: remove need for a temporary string for the node name

Currently the node name is being formatting into a temporary string
node_name, however, kobject_init_and_add allows one to format up
a node name, so use that instead. This removes the need for the
node_name string and also cleans up the following warning:

Fixes clang warning:
warning: format string is not a string literal (potentially
insecure) [-Wformat-security]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>