s390/qeth: remove QETH_RX_PULL_LEN

Since commit f677fcb9 ("s390/qeth: ensure linear access to packet headers"),
the CQ-specific skbs are allocated with a slightly bigger linear part
than necessary. Shrink it down to the maximum that's needed by
qeth_extract_skb().

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