mlx5, calc_sq_size(): Make a debug message more informative

Make it clear that qp->sq.wqe_cnt is not the number of WQEs.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Eli Cohen <eli@mellanox.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>