mt76: testmode: remove undefined behaviour in mt76_testmode_alloc_skb

Get rid of an undefined behaviour in mt76_testmode_alloc_skb routine
allocating skb frames

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>