mlx4_core: Fix state check in mlx4_qp_modify()

When checking the states passed in, mlx4_qp_modify() accidentally checks
cur_state twice rather than checking cur_state and new_state.  Fix this
to make sure that both values are in-bounds.

Since these values may be passed in from userspace, this bug results in
userspace being able to trigger an oops.

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