net: phylib: fix interrupts re-enablement in phy_start

If phy was suspended and is starting, current driver always enable
phy's interrupts, if phy works in polling, phy can raise unexpected
interrupt which will not be handled, the interrupt will block system
enter suspend again. So interrupts should only be re-enabled if phy
works in interrupt.

Signed-off-by: Shaohui Xie <Shaohui.Xie@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>