net: reduce indentation of __dev_alloc_name()

All callers of __dev_valid_name() go thru dev_prep_valid_name()
which handles the non-printf case. Focus __dev_alloc_name() on
the sprintf case, remove the indentation level.

Minor functional change of returning -EINVAL if % is not found,
which should now never happen.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20231023152346.3639749-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>