net/9p/usbg: fix CONFIG_USB_GADGET dependency

When USB gadget support is in a loadable module, 9pfs cannot
link to it as a built-in driver:

x86_64-linux-ld: vmlinux.o: in function `usb9pfs_free_func':
trans_usbg.c:(.text+0x1070012): undefined reference to `usb_free_all_descriptors'
x86_64-linux-ld: vmlinux.o: in function `disable_ep':
trans_usbg.c:(.text+0x1070528): undefined reference to `usb_ep_disable'
x86_64-linux-ld: vmlinux.o: in function `usb9pfs_func_unbind':
trans_usbg.c:(.text+0x10705df): undefined reference to `usb_ep_free_request'
x86_64-linux-ld: trans_usbg.c:(.text+0x107061f): undefined reference to `usb_ep_free_request'
x86_64-linux-ld: vmlinux.o: in function `usb9pfs_func_bind':
trans_usbg.c:(.text+0x107069f): undefined reference to `usb_interface_id'
x86_64-linux-ld: trans_usbg.c:(.text+0x10706b5): undefined reference to `usb_string_id'

Change the Kconfig dependency to only allow this to be enabled
when it can successfully link and work.

Fixes: a3be076d ("net/9p/usbg: Add new usb gadget function transport")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240909111745.248952-1-arnd@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>