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 2193ede1 ("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 <m.grzeschik@pengutronix.de>
Link: https://lkml.kernel.org/r/ZzhWRPDNwu225NWz@codewreck.org


Message-ID: <20241122144754.1231919-1-asmadeus@codewreck.org>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>