IB/mthca: Don't double-free IRQs when falling back from MSI-X to INTx

When both MSI-X and legacy INTx fail to generate an interrupt, the
driver frees the MSI-X interrupts twice.  Fix this by clearing the
have_irq flag for the MSI-X interrupts when they are freed the first
time.

Reported-by: Yinghai Lu <yhlu.kernel@gmail.com>
Tested-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>