qeth: Clear SBALF15 in any case for output buffers.

Function qeth_clear_output_buffer for HiperSockets may not clear
all 16 SBALEs, but only the used ones. The error flag in SBALF15
has to be cleared in any case.

Signed-off-by: Klaus-Dieter Wacker <kdwacker@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>