sfc: Remove workaround for old firmware bug

There was a bug in XAUI synchronisation in early 10Xpress firmware
versions.  This is fixed in released firmware and we do not need to
work around it.

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