RDMA/cxgb4: re-fix 32-bit build warning

Casting a pointer to __be64 produces a warning on 32-bit architectures:

drivers/infiniband/hw/cxgb4/mem.c:147:20: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
    req->wr.wr_lo = (__force __be64)&wr_wait;

This was fixed at least twice for this driver in different places,
and accidentally reverted once more. This puts the correct version
back in place.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 6198dd8d ("iw_cxgb4: 32b platform fixes")
Signed-off-by: Doug Ledford <dledford@redhat.com>