devlink: make devlink_flash_overwrite enum named one

Since this enum is going to be used in generated userspace file, name it
properly.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20231021112711.660606-7-jiri@resnulli.us


Signed-off-by: Jakub Kicinski <kuba@kernel.org>