IB/ipath: Fix check for no interrupts to reliably fallback to INTx

Newer HCAs support MSI interrupts and also INTx interrupts.  Fix the
code so that INTx can be reliably enabled if MSI interrupts are not
working.

Signed-off-by: Dave Olson <dave.olson@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>