mlx5: Use enum to indicate adapter page size

mlx5: Use enum to indicate adapter page size

The Connect-IB adapter has an inherent page size which equals 4K.
Define an new enum that equals the page shift and use it instead of
using the value 12 throughout the code.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>