IB/mthca: Fix mthca_array_clear() thinko

mthca_array_clear() does not clear the slot if the used count is
positive. This leads to crashes in mthca_qp_event() since that uses
mthca_array_get() to check that the qp is valid.

Discovered by Ali Ayoub.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>