net: usb: initialize tmp in dm9601.c to avoid warning

In two places, tmp is initialized implicitly by being passed as a
pointer during a function call.  However, this is not obvious to the
compiler, which logs a warning.

Signed-off-by: Simon Que <sque@chromium.org>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>