de2104x: remove BUG_ON() when changing media type

When the chip dies (probably because of a bug somewhere in the driver),
de_stop_rxtx() fails and changing the media type crashes the whole machine.
Replace BUG_ON() in de_set_media() with a warning.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>