IB/core: Set hop limit in ib_init_ah_from_wc correctly

The hop_limit value in the ah_attr should be 0xFF, not the value read
from the received GRH (which should be 0).  See 13.5.4.4 in the 1.2 IB
spec.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>