benet: use do_div() for 64 bit divide

The benet driver is doing a 64 bit divide, which is not supported in
Linux kernel on 32 bit architectures. The correct way to do this is to
use do_div().  Compile tested on i386 only.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>