IB/mthca: Set GRH:HopLimit when building MLX headers

Global CM packets used by rmda_cm were being sent with a GRH:hopLimit
of zero, causing them to be dropped by the router.  The problem is a
missing initialization of the hop_limit field in mthca_read_ah(),
which was called by build_mlx_header() when sending a MAD on QP1.

Signed-off-by: Rolf Manderscheid <rvm@obsidianresearch.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>