net: au1000_eth: Mark au1000_ReleaseDB() static

This fixes the following build warning:
```
drivers/net/ethernet/amd/au1000_eth.c:574:6: warning: no previous prototype for 'au1000_ReleaseDB' [-Wmissing-prototypes]
574 | void au1000_ReleaseDB(struct au1000_private *aup, struct db_dest *pDB)
| ^~~~~~~~~~~~~~~~
```

Signed-off-by: Johan Korsnes <[e-mail]>
Cc: Andrew Lunn <[e-mail]>
Reviewed-by: Michal Swiatkowski <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/amd/au1000_eth.c

···
 	return pDB;
 }
 
-void au1000_ReleaseDB(struct au1000_private *aup, struct db_dest *pDB)
+static void au1000_ReleaseDB(struct au1000_private *aup, struct db_dest *pDB)
 {
 	struct db_dest *pDBfree = aup->pDBfree;
 	if (pDBfree)