cxgb4vf: fix TX Queue restart

Fix up TX Queue Host Flow Control to cause an Egress Queue Status Update to
be generated when we run out of TX Queue Descriptors.  This will, in turn,
allow us to restart a stopped TX Queue.

Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>