cxgb4: Fix tx flit calculation

In commit 0aac3f56

 ("cxgb4: Add comment for calculate tx flits
and sge length code") introduced a regression where tx flit calculation
is going wrong, which can lead to data corruption, hang, stall and
write-combining failure. Fixing it.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>