Fix USB gadget net2280.c compile

It would fail the compile due to the newly added error checking testing
a bad macro for a "return value" unless USB_GADGET_DEBUG_FILES was
enabled.

Pointed out by Stephen Hemminger.

Acked-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>