ieee802154: correct ieee802154_is_valid_psdu_len

This patch corrects the ieee802154_is_valid_psdu_len function that this
function also checks on reserved values 6-8 for validation the psdu
length.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>