scripts: kernel-doc: support in-line comments on nested structs/unions

The parser at kernel-doc rejects names with dots in the middle.
Fix it, in order to support nested structs/unions.

Tested-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>