wireguard: receive: use dev_sw_netstats_rx_add()

We have a convenient helper, let's use it.
This will make the following patch easier to review and smaller.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>