qeth: change some configurations defaults

This patch turns on RX checksum and GRO by default. To improve
receiving performance and reduce congestion in case of network
bursts we also increase the default number of inbound buffers.

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