mac802154: tx: use put_unaligned_le16 for copy crc

This patch replaces the memcpy with a put_unaligned_le16. The placement
of crc inside of PSDU can also be unaligned. With memcpy this can fail
on some architectures.

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