mlx4_core: Fix thinko in QP destroy (incorrect bitmap_free)

Fix thinko in commit eaf559bf

 ("mlx4_core: Don't free special QPs in
QP number bitmap").  The old commit had the logic exactly backwards
and ended up freeing *only* special QPs, which not only left the
original bug in place but also introduced the problem that the QP
number bitmap would get full after a while.

Found by Dotan Barak of Mellanox.

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