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 <johan.korsnes@gmail.com>
Cc: Andrew Lunn <andrew+netdev@lunn.ch>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Link: https://patch.msgid.link/20250323190450.111241-1-johan.korsnes@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: johankor

drivers/net/ethernet/amd/au1000_eth.c

@@ -571,7 +571,7 @@ static struct db_dest *au1000_GetFreeDB(struct au1000_private *aup)
 	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)