drivers: net: cpsw: dual_emac: simplify napi usage

Since interrupt is shared between the two ethernet interface and
in isr only one napi is scheduled at an instance so having two
napis doesn't make any difference. So making napi also as a
common resource for the dual ethernet interfaces.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>