sh_eth: use DIV_ROUND_UP() in sh_eth_soft_swap()

When initializing 'maxp' in sh_eth_soft_swap(), the buffer length needs
to be rounded  up -- that's just asking for DIV_ROUND_UP()!

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>