cpmac: prevent fatal exception in cpmac_end_xmit

We should not be stopping the subqueues in cpmac_end_xmit
but rather test the status of them. Replace the calls to
netif_subqueue_stop by __netif_subqueue_stopped. This
fixes an unrecoverable exception from happening when
running the driver.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>