sfc: Use separate hardware TX queues to select checksum generation

Checksum generation is an attribute of our hardware TX queues, not TX
descriptors.  We previously used a single queue and turned checksum
generation on or off as requested through ethtool.  However, this can
result in regenerating checksums in raw packets that should not be
modified.  We now create 2 hardware TX queues with checksum generation
on or off.  They are presented to the net core as one queue since it
does not know how to select between them.

The self-test verifies that a bad checksum is unaltered on the queue
with checksum generation off.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>