RDMA/cxgb4: Fix underflows in c4iw_create_qp()

These sizes should be unsigned so we don't allow negative values and
have underflow bugs.  These can come from the user so there may be
security implications, but I have not tested this.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>