net: microchip: encx24j600: add missed kthread_stop

This driver calls kthread_run() in probe, but forgets to call
kthread_stop() in probe failure and remove.
Add the missed kthread_stop() to fix it.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>