mlx5_core: Adjust hca_cap.uar_page_sz to conform to Connect-IB spec

Sparse reported an endianness bug in the assignment to hca_cap.uar_page_sz.

Fix the declaration of this field to be __be16 (which is what is in
the firmware spec), renaming the field to log_uar_pg_size to conform
to the spec, which fixes the endianness bug reported by sparse.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Moshe Lazer <moshel@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>