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>