mt76: testmode: fix memory leak in mt76_testmode_alloc_skb

Free all pending frames in case of failure in mt76_testmode_alloc_skb
routine

Fixes: 2601dda8 ("mt76: testmode: add support to send larger packet")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>