[IB] mthca: Always re-arm EQs in mthca_tavor_interrupt()

We should always re-arm an event queue's interrupt in
mthca_tavor_interrupt() if the corresponding bit is set in the event
cause register (ECR), even if we didn't find any entries in the EQ.
If we don't, then there's a window where we miss an EQ entry and then
get stuck because we don't get another EQ event.

Signed-off-by: Roland Dreier <rolandd@cisco.com>