s390/qeth: consolidate pack buffer flushing

qeth_switch_to_nonpacking_if_needed() contains an open-coded version
of qeth_flush_buffers_on_no_pci(). Extract a single helper instead.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Acked-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>