s390/qeth: pass TSO header length to fill_buffer()

The TSO code already calculates the length of its header element,
no need to duplicate this in the low-level code again.

Use this opportunity to make hd_len unsigned, and for TSO match
its calculation to what tso_fill_header() does.

No functional change.

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