s390/qeth: update kerneldoc for qeth_add_hw_header()

qeth_add_hw_header() is missing documentation for some of its
parameters, fix that up.

Reported-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>