r8169: don't read interrupt mask register in interrupt handler

After the original patch network starts to crash on heavy load.
It's not fully clear why this additional register read has such side
effects, but removing it fixes the issue.

Thanks also to Alex for his contribution and hints.

[0] https://marc.info/?t=155268170400002&r=1&w=2

Fixes: e782410e

 ("r8169: improve spurious interrupt detection")
Reported-by: VDR User <user.vdr@gmail.com>
Tested-by: VDR User <user.vdr@gmail.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>