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

[ Upstream commit 1adb20f0 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I75b9917ff04176552dfb2a6e35d14b54b4e743d5