r8169: merge rtl_irq_enable and rtl_irq_enable_all

After the recent changes to the interrupt handler rtl_irq_enable and
rtl_irq_enable_all can be merged.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>