hw/usb-net.c: Fix precedence bug when checking rndis_state

"!X == 2" is always false (spotted by Coverity), so the checks
for whether rndis is in the correct state would never fire.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>
