scripts/kernel-doc: make unknown function prototype a Warning instead of an Error

When scripts/kernel-doc cannot understand a function prototype,
it had been generating a fatal error and stopping immediately.
Make this a Warning instead of an Error and keep going.

Note that this can happen if the kernel-doc notation that is being
parsed is not actually a function prototype; maybe it's a struct or
something else, so I added "function" to the warning message to try
to make it clearer that scripts/kernel-doc is looking for a function
prototype here.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc:	Mark Brown <broonie@kernel.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/kernel-doc

@@ -2125,8 +2125,7 @@ sub dump_function($$) {
 
 	create_parameterlist($args, ',', $file);
     } else {
-	print STDERR "Error(${file}:$.): cannot understand prototype: '$prototype'\n";
-	++$errors;
+	print STDERR "Warning(${file}:$.): cannot understand function prototype: '$prototype'\n";
 	return;
     }