net: dsa: hellcreek: Check return value

Return NULL if we got unexpected value from skb_trim_rcsum()
in hellcreek_rcv()

Fixes: 01ef09ca

 ("net: dsa: Add tag handling for Hirschmann Hellcreek switches")
Signed-off-by: Artem Chernyshev <artem.chernyshev@red-soft.ru>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Link: https://lore.kernel.org/r/20221201140032.26746-2-artem.chernyshev@red-soft.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>