s390/qeth: fix promiscuous mode after reset

When managing the promiscuous mode during an RX modeset, qeth caches the
current HW state to avoid repeated programming of the same state on each
modeset.

But while tearing down a device, we forget to clear the cached state. So
when the device is later set online again, the initial RX modeset
doesn't program the promiscuous mode since we believe it is already
enabled.
Fix this by clearing the cached state in the tear-down path.

Note that for the SBP variant of promiscuous mode, this accidentally
works right now because we unconditionally restore the SBP role while
re-initializing.

Fixes: 4a71df50 ("qeth: new qeth device driver")
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>