net: stmmac: dwmac1000: Fix extended MAC address registers definition

The register starts from 0x800 is the 16th MAC address register rather
than the first one.

Fixes: cffb13f4

 ("stmmac: extend mac addr reg and fix perfect filering")
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: David S. Miller <davem@davemloft.net>