IB/mlx5: Simplify WQE count power of two check

Use is_power_of_2() instead of hard coding it in the driver. While at it,
fix the meaningless error print.

Signed-off-by: Gal Pressman <galpress@amazon.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>