net: ethernet: cortina: No mapping is a dropped rx

net: ethernet: cortina: No mapping is a dropped rx

Increase stats.rx_dropped++ even if this is the first fragment
(skb == NULL) so we are doing proper accounting.

Fixes: b266bacba796 ("net: ethernet: cortina: Drop half-assembled SKB")
Link: https://sashiko.dev/#/patchset/20260505-gemini-ethernet-fix-v2-1-997c31d06079%40kernel.org
Signed-off-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/20260509-gemini-ethernet-fixes-v1-1-6c5d20ddc35b@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>