net: chelsio: cxgb4vf: Fix wrong function name in comments

Fixes the following W=1 kernel build warning(s):

 drivers/net/ethernet/chelsio/cxgb4vf/sge.c:966: warning: expecting prototype for check_ring_tx_db(). Prototype was for ring_tx_db() instead

Cc: Raju Rangoju <rajur@chelsio.com>
Signed-off-by: Yang Shen <shenyang39@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>