IB/mthca: Fix access to freed memory in catastrophic event handling

catas_reset() uses a pointer to mthca_dev, but mthca_dev is not valid
after the call to __mthca_restart_one().

Based on a similar patch for mlx4 (634354d7, "mlx4: Fix access to
freed memory") by Vitaliy Gusev <vgusev@openvz.org>

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>