usb: gadget: Kconfig: Fix configfs-based RNDIS function build

USB_CONFIGFS_RNDIS depends on USB_U_RNDIS. Select it.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>