ftgmac100: Restart MAC HW once

The interrupt handler may set the flag to reset the mac in the future,
but that flag is not cleared once the reset has occurred.

Fixes: 10cbd640

 ("ftgmac100: Rework NAPI & interrupts handling")
Signed-off-by: Dylan Hung <dylan_hung@aspeedtech.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: David S. Miller <davem@davemloft.net>