cxgb4: when max_tx_rate is 0 disable tx rate limiting

in ndo_set_vf_rate() when max_tx_rate is 0 disable tx
rate limiting for that vf.

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