net/ethernet: cpsw: Bugfix interrupts before enabling napi

If interrupts happen before napi_enable was called, the driver will not
work as expected. Network transmissions are impossible in this state.
This bug can be reproduced easily by restarting the network interface in
a loop. After some time any network transmissions on the network
interface will fail.

This patch fixes the bug by enabling napi before enabling the network
interface interrupts.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>