mlx5en(4): Use hard-coded 4K page size for RQ/SQ/CQ.

The page size specified for RQ, SQ and CQ is always in units of 4KBytes.
Make sure we subtract MLX5_ADAPTER_PAGE_SHIFT, 12, instead of PAGE_SHIFT
which may vary. This fixes support for using the mlx5en driver on systems
having non-4K page size.

Linux commit:
68cdf5d6e91068c98d6091b193dc7a5ab7dcf5eb

MFC after: 1 week
Sponsored by: NVIDIA Networking