net/mlx4_en: Improve transmit CQ polling

net/mlx4_en: Improve transmit CQ polling

Several small performance improvements in TX CQ polling,
including:
- Compiler branch predictor hints.
- Minimize variables scope.
- More proper check of cq type.
- Use boolean instead of int for a binary indication.

Performance tests:
Tested on ConnectX3Pro, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz

Packet-rate tests for both regular stack and XDP use cases:
No noticeable gain, no degradation.

Signed-off-by: Tariq Toukan <[email protected]>
Reviewed-by: Saeed Mahameed <[email protected]>
Cc: [email protected]
Cc: Eric Dumazet <[email protected]>
Signed-off-by: David S. Miller <[email protected]>