usb: gadget: zero: fix bug in loopback autoresume handling

ab943a2e (USB: gadget: gadget zero uses new suspend/resume hooks)
introduced a copy-paste error where f_loopback.c writes to a variable
declared in f_sourcesink.c. This prevents one from creating gadgets
that only have a loopback function.

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Timo Juhani Lindfors <timo.lindfors@iki.fi>
Signed-off-by: Felipe Balbi <balbi@ti.com>