s390/qeth: streamline control code for promisc mode

We have logic to determine the desired promisc mode in _each_ code path.
Change things around so that there is a clean split between
(a) high-level code that selects the new mode, and (b) implementations
of the various mechanisms to program this mode.

This also keeps qeth_promisc_to_bridge() from polluting the debug logs
on each RX modeset.

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>