qeth: core code should alloc headroom for LLC protocol

Allocate headroom for TR_HLEN but using only ETH_HLEN causes rx
performance degradation. Allocate ETH_HLEN for ethernet and
TR_HLEN for token ring (layer 3 mode).

Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>