net/mlx5e: Present SW stats when state is not opened

The driver can present all SW stats even when the state not opened.
Fixed get strings, count and stats to support it.

In addition, fix tc2txq to hold a static mapping which doesn't depend on
the amount of open channels, and cannot have the same value on two
different cells  while moving between configurations.
Example:
- OOB 16 channels
- Change to 2 channels, 8 TCs
- tc2txq[15][0] == tc2txq[1][7] == 15
This will cause multiple appearances of the same TX index in statistics
output.

Fixes: 76c3810bade3 ("net/mlx5e: Avoid reset netdev stats on configuration changes")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>