chelsio: tabulate the update of the statistic counters

Let's try to avoid some code duplication.

- cxgb2
  The data are contiguous. Use plain memcpy.

- ixf1010/pm3393/vsc7326
  The cast of &mac->stats to (u64 *) is not wonderful but it is not clear
  if it is worth to add an ad-hoc union under the struct cmac_statistics.

vsc7326_reg.h suggests that more statistics could be available.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>