s390/qeth: shift some bridgeport code around

qeth_l2_setup_bridgeport_attrs() is entirely unrelated to sysfs
functionality, move it where it belongs.
While at it merge all the bridgeport-specific code in the set-online
path together.

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