net: natsemi: fix `rx_dropped` double accounting on `netif_rx()` failure

`netif_rx()` already increments `rx_dropped` core stat when it fails.
The driver was also updating `ndev->stats.rx_dropped` in the same path.
Since both are reported together via `ip -s -s` command, this resulted
in drops being counted twice in user-visible stats.

Keep the driver update on `if (unlikely(!skb))`, but skip it after
`netif_rx()` errors.

Fixes: caf586e5 ("net: add a core netdev->rx_dropped counter")
Signed-off-by: Yeounsu Moon <yyyynoom@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250913060135.35282-3-yyyynoom@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>