firewire: cdev: require quadlet-aligned headers for transmit packets

The definition of struct fw_cdev_iso_packet seems to imply that the
header_length must be quadlet-aligned, and in fact, specifying an
unaligned header has never really worked when using multiple packet
structures, because the position of the next control word is computed by
rounding the header_length _down_, so the last one to three bytes of the
header would overlap the next control word.

To avoid this problem, check that the header length is properly aligned.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>