net: dsa: sja1105: Check return value

Return NULL if we got unexpected value from skb_trim_rcsum() in
sja1110_rcv_inband_control_extension()

Fixes: 4913b8eb

 ("net: dsa: add support for the SJA1110 native tagging protocol")
Signed-off-by: Artem Chernyshev <artem.chernyshev@red-soft.ru>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20221201140032.26746-3-artem.chernyshev@red-soft.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>