net/mlx5: EQ, Move all EQ logic to eq.c

Move completion EQs flows from main.c to eq.c, reasons:
1) It is where this logic belongs.
2) It will help centralize the EQ logic in one file for downstream
refactoring, and future extensions/updates.

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>