hw/net/pcnet: Use qemu_log_mask(GUEST_ERROR) instead of printf

Avoid to clutter stdout until explicitly requested
(with -d guest_errors):

  $ qemu-system-mips -M malta -m 512 -kernel vmlinux-3.2.0-4-4kc-malta
  Bad SWSTYLE=0x04

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190311102712.8572-1-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/net/pcnet.c

@@ -36,6 +36,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/log.h"
 #include "hw/qdev.h"
 #include "net/net.h"
 #include "net/eth.h"
@@ -1501,7 +1502,8 @@ static void pcnet_bcr_writew(PCNetState *s, uint32_t rap, uint32_t val)
             val |= 0x0300;
             break;
         default:
-            printf("Bad SWSTYLE=0x%02x\n", val & 0xff);
+            qemu_log_mask(LOG_GUEST_ERROR, "pcnet: Bad SWSTYLE=0x%02x\n",
+                          val & 0xff);
             val = 0x0200;
             break;
         }