net: ethernet: ti: cpsw: update cpsw_split_res() to accept cpsw_common

Update cpsw_split_res() to accept struct cpsw_common instead of
struct net_device to simplify code.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>