net/mlx5e: Xmit, no write combining

mlx5e netdev Blue Flame (write combining) support demands a lot of
overhead for a little latency gain for some special cases, this overhead
is hurting the common case.

Here we remove xmit Blue Flame support by creating all bfregs with no
write combining for all SQs, and we remove a lot of BF logic and
conditions from xmit data path.

Simplify mlx5e_tx_notify_hw (doorbell function) by removing BF related
code and by removing one memory barrier needed for WC mapped SQ doorbell
buffers, which no longer exist.

Performance improvement:
System: Intel(R) Xeon(R) CPU E5-2620 v3 @ 2.40GHz

Test case                   Before      Now      improvement
---------------------------------------------------------------
TX packets (24 threads)     50Mpps      54Mpps    8%

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>