net/ethernet: There is no need to fill in the header in all frags

Only the first one requires it. Actually drivers know that already and
handle the frags list correctly.

In case ethernet has to run along with 15.4 on the same SoC, this will
optimize things quite a bit knowing that biggest ethernet frame will be
forcefully split in as many 128 bytes frags as necessary.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>