s390/qeth: Consider dependency on SWITCHDEV module

Without the SWITCHDEV module, the bridgeport attribute LEARNING_SYNC
of the physical device (self) does not provide any functionality.
Instead of calling the no-op stub version of the switchdev functions,
fail the setting of the attribute with an appropriate message.

While at it, also add an error message for the 'not supported by HW'
case.

Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>