net: stmmac: Fix "Unbalanced pm_runtime_enable!" warning

If the device is PCI based like intel-eth-pci, pm_runtime_enable() is
already called by pci_pm_init().

So only pm_runtime_enable() when it's not already enabled.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>