net/sfc: fix MAC stats update for stopped device

[ upstream commit 1827b073 ]

Return the latest stats snapshot in stopped state
instead of returning an error.

Fixes: 1caab2f1 ("net/sfc: add basic statistics")

Signed-off-by: Ivan Ilchenko <ivan.ilchenko@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>