net/9p/usbg: allow building as standalone module

There is no reason only the usbg transport would not be its own module,
so make it tristate.

In particular, this fixes a couple of issues the current bool had:
- trans_usbg was apparently not compiled at all when NET_9P=m
- the workaround added in commit 2193ede180dd ("net/9p/usbg: fix
CONFIG_USB_GADGET dependency") became redundant because a tristate item
cannot be built-in when its dependency is a module, so we can depend on
USB_GADGET "normally" again.

Cc: Michael Grzeschik <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]
Message-ID: <[e-mail]>
Signed-off-by: Dominique Martinet <[e-mail]>

net/9p/Kconfig

···
 	  two Xen domains.
 
 config NET_9P_USBG
-	bool "9P USB Gadget Transport"
-	depends on USB_GADGET=y || USB_GADGET=NET_9P
+	tristate "9P USB Gadget Transport"
+	depends on USB_GADGET
 	select CONFIGFS_FS
 	select USB_LIBCOMPOSITE
 	help