usb: gadget: webcam: fix V4L2 Kconfig dependency

Configuring the USB_G_WEBCAM driver as built-in leads to a link
error when CONFIG_VIDEO_V4L2 is a loadable module:

drivers/usb/gadget/function/f_uvc.o: In function `uvc_function_setup':
f_uvc.c:(.text+0xfe): undefined reference to `v4l2_event_queue'
drivers/usb/gadget/function/f_uvc.o: In function `uvc_function_ep0_complete':
f_uvc.c:(.text+0x188): undefined reference to `v4l2_event_queue'

This changes the Kconfig dependency to disallow that configuration,
and force it to be a module in that case as well.

This is apparently a rather old bug, but very hard to trigger
even in thousands of randconfig builds.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>