usbip: vudc: fix Kconfig dependencies

With the addition of VUDC, the USBIP stack can now be used on
configurations without USB host support, but trying to build
it with USB gadget support disabled fails with

drivers/usb/built-in.o: In function `vep_dequeue':
vudc_main.c:(.text+0xa6ddc): undefined reference to `usb_gadget_giveback_request'
drivers/usb/built-in.o: In function `nuke':
vudc_main.c:(.text+0xa6ea8): undefined reference to `usb_gadget_giveback_request'
drivers/usb/built-in.o: In function `vudc_device_reset':
vudc_main.c:(.text+0xa720c): undefined reference to `usb_gadget_udc_reset'
drivers/usb/built-in.o: In function `vudc_probe':

This addresses both issues, by changing the dependency for USBIP_CORE
to USB_COMMON, and adding additional dependencies on USB or USB_GADGET
for the individual portions as needed.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 9360575c ("usbip: vudc: Add vudc to Kconfig")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>