IB/mthca: Initialize grh_present before using it

build_mlx_header() was using sqp->ud_header.grh_present before it was
initialized by mthca_read_ah().  Furthermore, header->grh_present is
set by ib_ud_header_init, so there's no need to set it again in
mthca_read_ah().

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>