RDMA/cxgb4: Fix gcc warning on 32-bit arch

RDMA/cxgb4: Fix gcc warning on 32-bit arch

Building mem.o for 32 bits x86 triggers a GCC warning:

    drivers/infiniband/hw/cxgb4/mem.c: In function '_c4iw_write_mem_dma_aligned':
    drivers/infiniband/hw/cxgb4/mem.c:79:25: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]

Silence that warning by casting "&wr_wait" to unsigned long before
casting it to __be64.  That's what _c4iw_write_mem_inline() already does.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>