ixgbe: Correctly obtain protocol information on transmit

In reply to "RE: [Fwd: [PATCH 2.6.25] ixgbe/igb: correctly obtain protocol
information on transmit]" from Andy Gospodarek:

The driver was incorrectly looking at socket headers for
protocol information, needed for checksumming offload. Fix
this by not looking at the socket but frame headers instead.

This disregards extension headers but it's unclear that linux
generates those anyway.

Tested by Andy Gospodarek.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>