net: ethernet: ti: cpsw: move napi struct to cpsw_common

The napi structs are common for both net devices in dual_emac
mode, In order to not hold duplicate links to them, move to
cpsw_common.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>