qeth: specify correct function level for OSN devices

OSN devices use the same function level as OSD devices. This patch
adds OSN-devices to the initialization function for func_level.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>