kernel-doc: allow unnamed bit-fields

Allow for unnamed bit-fields and skip them instead of printing an
erroneous warning message for them, such as:

Warning(include/asm-s390/cio.h:103): No description found for parameter 'u32'

which contains:

struct tm_scsw {
	u32 :1;
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/kernel-doc

@@ -1556,8 +1556,10 @@ sub create_parameterlist($$$) {
 		    push_parameter($2, "$type $1", $file);
 		}
 		elsif ($param =~ m/(.*?):(\d+)/) {
+		    if ($type ne "") { # skip unnamed bit-fields
 			push_parameter($1, "$type:$2", $file)
 		    }
+		}
 		else {
 		    push_parameter($param, $type, $file);
 		}