net: dsa: ksz: Check return value

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

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: bafe9ba7

 ("net: dsa: ksz: Factor out common tag code")
Signed-off-by: Artem Chernyshev <artem.chernyshev@red-soft.ru>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20221201140032.26746-1-artem.chernyshev@red-soft.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>