mac802154: Fix possible double free upon parsing error

Commit 4d1c7d87 ("mac802154: Move an skb free within the rx path")
tried to simplify error handling within the receive path by moving the
kfree_skb() call at the very end of the top-level function but missed
one kfree_skb() called upon frame parsing error. Prevent this possible
double free from happening.

Fixes: 4d1c7d87

 ("mac802154: Move an skb free within the rx path")
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20221216235742.646134-1-miquel.raynal@bootlin.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>