mlx5_core: Fix out arg size in access_register command

The output size should be the sum of the core access reg output struct
plus the size of the specific register data provided by the caller.

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