s390/qeth: clean up default cases for ethtool link mode

Remove the default case for PORT_* and SPEED_* in our ethtool code.

The only time these could be hit is if qeth_init_link_info() was unable
to determine the port type from an OSA adapter's link_type.
We already throw a message in this case, so reduce the noise and don't
report bad data (ie. it's much more likely that any future link_type
will represent a PORT_FIBRE link ...).

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>