net/mlx5: E-Switch, Rename esw attr mirror count field

The mirror count esw attributes field is used to determine if splitting
the rule to two FTEs is required while programming e-switch mirroring.
Rename it to split count, making it clearer with no functional change.

Signed-off-by: Eli Britstein <elibr@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>