net: stmmac: xgmac: Fix XGMAC selftests

Fixup the XGMAC selftests by correctly finishing the implementation of
set_filter callback.

Result:
$ ethtool -t enp4s0
The test result is PASS
The test extra info:
 1. MAC Loopback         	 0
 2. PHY Loopback         	 -95
 3. MMC Counters         	 -95
 4. EEE                  	 -95
 5. Hash Filter MC       	 0
 6. Perfect Filter UC    	 0
 7. MC Filter            	 0
 8. UC Filter            	 0
 9. Flow Control         	 0

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>