usbnet: smsc95xx: simplify tx_fixup code

The smsc95xx_tx_fixup is doing multiple calls to skb_push() to
put an 8-byte command header onto the packet. It would be easier
to do one skb_push() and then copy the data in once the push is
done.

We also make the code smaller by using proper unaligned puts for
the header. This merges in the CPU to LE32 conversion as well and
makes the whole sequence easier to understand hopefully.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>