cxgb4: Fix an error code in cxgb4_mqprio_alloc_hw_resources()

"ret" is zero or possibly uninitialized on this error path.  It
should be a negative error code instead.

Fixes: 2d0cb84d

 ("cxgb4: add ETHOFLD hardware queue support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>