r8169: improve rtl8169_irq_mask_and_ack

Code can be slightly simplified by acking even events we're not
interested in. In addition add a comment making clear that the
read has no functional purpose and is just a PCI commit.

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