net: core: dev: replace state xoff flag comparison by netif_xmit_stopped method

Function netif_schedule_queue() has a hardcoded comparison between queue
state and any xoff flag. This comparison does the same thing as method
netif_xmit_stopped(). In terms of code clarity, it is better. See other
methods like: generic_xdp_tx() and dev_direct_xmit().

Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>