r8169: fix RxMissed register access

- the register is defined for the 8169 chipset only and there is
  no 8169 beyond RTL_GIGA_MAC_VER_06.
- only the lower 3 bytes of the register are valid

Fixes:
1. http://bugzilla.kernel.org/show_bug.cgi?id=10180
2. http://bugzilla.kernel.org/show_bug.cgi?id=11062

 (bits of)

Tested by Hermann Gausterer and Adam Huffman.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Edward Hsu <edward_hsu@realtek.com.tw>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>