wifi: mt76: mt76x02: fix return value check in mt76x02_mac_process_rx

in mt76x02_mac_process_rx(), return an error code if an
unexpected result is returned by pskb_trim.

Signed-off-by: Yuanjun Gong <ruc_gongyuanjun@163.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>