mlx5: Clear reserved area in set_hca_cap()

Firmware spec requires reserved fields to be cleared when calling
set_hca_cap.  Current code queries and copy to the set area, possibly
resulting in reserved bits not cleared. This patch copies only
writable fields to the set area.

Fix also typo - msx => max

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