common/mlx5: update doorbell mapping parameter name

common/mlx5: update doorbell mapping parameter name

The "tx_db_nc" devarg forces doorbell register mapping to non-cached
region eliminating the extra write memory barrier. This argument was
used in creating the UAR for Tx and thus affected its performance.

Recently [1] its use has been extended to all UAR creation in all mlx5
drivers, and now its name is no longer so accurate.

This patch changes its name to "sq_db_nc" to suit any send queue that
uses it. The old name will still work for backward compatibility.

[1] commit 5dfa003db53f ("common/mlx5: fix post doorbell barrier")

Signed-off-by: Michael Baum <michaelba@nvidia.com>
Reviewed-by: Raslan Darawsheh <rasland@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>