net/mlx5: Make get_cqe routine not ethernet-specific

Move mlx5e_get_cqe routine to wq.h and rename it to
mlx5_cqwq_get_cqe.

This allows it to be used by other CQ users outside of the
ethernet driver code.

A later patch in this patchset will make use of it from
FPGA code for the FPGA high-speed connection.

Signed-off-by: Ilan Tayari <ilant@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>