e1000: Fix multi-descriptor packet checksum offload

The PCI/PCI-X Family of Gigabit Ethernet Controllers Software
Developer’s Manual states the following about the POPTS field:

  Provides a number of options which control the handling of this
  packet.  This field is ignored except on the first data descriptor of
  a packet.

The current implementation always loads the field and its checksum
offload flags.  This patch uses only the first descriptor's POPTS field
in order to comply with the specification.

When Solaris sends multi-descriptor packets it fills in POPTS for the
first descriptor only.  Therefore this patch is necessary in order to
perform checksum offload correctly for multi-descriptor packets.

Reported-by: Daniel Pecka <dpecka@techniservit.cz>
Reported-by: Gabriele A. Trombetti <gabriele.trombetti@itb.cnr.it>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>