sfc/siena: Rename functions in nic_common.h to avoid conflicts with sfc

For siena use efx_siena_ as the function prefix.
efx_nic_update_stats_atomic is only used in efx_common.c, so move
it there.
efx_nic_copy_stats is not used in Siena, so it is removed.

Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>