arcnet: com20020: add enable and disable device on open/close

This patch changes the driver to properly work with the linux netif
interface. The controller gets enabled on open and disabled on close.
Therefor it removes every bogus start of the xceiver. It only gets
enabled on com20020_open and disabled on com20020_close.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>