usb, gadget: use appropriate warning accessors

Use the appropriate WARN() and WARN_ON() accessors to avoid a build error
when CONFIG_BUG=n:

	drivers/usb/gadget/configfs.c: In function 'config_usb_cfg_unlink':
	drivers/usb/gadget/configfs.c:442:2: error: implicit declaration of function '__WARN_printf'
	drivers/usb/gadget/configfs.c: In function 'configfs_do_nothing':
	drivers/usb/gadget/configfs.c:733:2: error: implicit declaration of function '__WARN'

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>