net/mlx5e: Reuse alloc cq code for all CQs allocation

Reuse the code for mlx5e_alloc_cq and mlx5e_alloc_drop_cq, as they
have a similar flow.

Prior to this patch, the CQEs in the "drop CQ" were not initialized,
fixed
it with the shared flow of alloc CQ.  This is not a critical bug as the
RQ connected to this CQ never moved to RTS, but still better to have
this right.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>