net/mlx5: EQ, No need to store eq index as a field

eq->index is used only for completion EQs and is assigned to be
the completion eq index, it is used only when traversing the completion
eqs list, and it can be calculated dynamically, thus remove the
eq->index field.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>