staging: sn9c102 depends on USB

If the USB code is a loadable module, this driver cannot
be built-in. This adds an explicit dependency on CONFIG_USB
so that Kconfig can force sn9c102 to be a module in this case.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Luca Risolia <luca.risolia@studio.unibo.it>
Cc: Mauro Carvalho Chehab <m.chehab@samsung.com>
Cc: linux-media@vger.kernel.org
Cc: linux-usb@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>