mlx4_core: Fix wrong offset in parsing query device caps response

The wrong offset was used when parsing the number of XRCs in
mlx4_QUERY_DEV_CAP().

Signed-off-by: Dotan Barak <dotanb@dev.mellanox.co.il>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>