usbmon can only be built as a module if usbcore is a module too. Trivial
changes to the relevant Kconfig and Makefile (and a few trivial changes
elsewhere) allow usbmon to be built as a module even if usbcore is
builtin.

This is verified to work in all 9 permutations (3 correctly prohibited
by Kconfig, 6 build a suitable result).
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>