cxgb3 - remove false positive in xgmac workaround

Qualify toggling of xgmac tx enable with not getting pause frames,
we might not make forward progress because the peer is sending
lots of pause frames.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>