sfc: Use __iowrite64_copy instead of a slightly different local function

__iowrite64_copy() isn't quite the same as efx_memcpy_64(), but
it looks close enough:

- The length is in units of qwords not bytes
- It never byte-swaps, but that doesn't make a difference now as PIO
  is only enabled for x86_64
- It doesn't include any memory barriers, but that's OK as there is a
  barrier just before pushing the doorbell
- mlx4_en uses it for the same purpose

Compile-tested only.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>