sfc: Use CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS where appropriate

For some buffers we use a starting offset of either NET_IP_ALIGN or 0
depending on whether we believe the architecture supports efficient
access to unaligned words.  There is now a config macro specifying
whether this is the case, so check that rather than checking for
specific architectures.

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