net: stmmac: dwxgmac: Account for the primary MAC address for UC filtering

net: stmmac: dwxgmac: Account for the primary MAC address for UC filtering

The same filter slots are used to store the main MAC address as well as
the address for the unicast filter. Let's account for that when deciding
whether or not to use promisc when programming the UC list in hardware.

Fixes: 0efedbf11f07 ("net: stmmac: xgmac: Fix XGMAC selftests")
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20260826140500.616466-5-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>