xilinx: Fix typo in comment (Marvel -> Marvell)

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/net/xilinx_axienet.c

@@ -98,7 +98,7 @@ static unsigned int tdk_read(struct PHY *phy, unsigned int req)
             r |= 1;
             break;
         case 17:
-            /* Marvel PHY on many xilinx boards.  */
+            /* Marvell PHY on many xilinx boards.  */
             r = 0x8000; /* 1000Mb  */
             break;
         case 18: