qeth: Reduce CPU consumption through less SIGA-r calls

Patch avoids SIGA-r calls in case of SIGA-r required. It only calls
SIGA-r if a threshold of free buffer is reached. CPU consumption is
reduced as a consequence.

Signed-off-by: Einar Lueck <elelueck@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>