RDMA/cxgb4: Make _c4iw_write_mem_dma() static

This patch marks the function _c4iw_write_mem_dma() as static
because it is not used outside this file, which fixes the warning:

    drivers/infiniband/hw/cxgb4/mem.c:176:5: warning: no previous prototype for ‘_c4iw_write_mem_dma’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>