IB/mthca: Convert FW commands to use wait_for_completion_timeout()

The kernel has had wait_for_completion_timeout() for a long time now.
mthca should use it to handle FW commands timing out, instead of
implementing the same thing in a much more complicated way by using
wait_for_completion() along with a timer that does complete().

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