cxgb4: Update proper netdev stats for rx drops

Count buffer group drops or truncates as rx drops rather than
rx errors in netdev stats.

Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: Arjun V <arjun@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>