usb: gadget: g_fs: Fix compilation warning

This commit fixes warning in f_fs.c introduced by "usb:
gadget: f_fs: remove custom printk() wrappers":

In file included from drivers/usb/gadget/g_ffs.c:64:
drivers/usb/gadget/f_fs.c:30:1: warning: "pr_fmt" redefined

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
diff --git a/drivers/usb/gadget/g_ffs.c b/drivers/usb/gadget/g_ffs.c
index 3dc1989..ebf6970 100644
--- a/drivers/usb/gadget/g_ffs.c
+++ b/drivers/usb/gadget/g_ffs.c
@@ -19,6 +19,8 @@
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
 
+#define pr_fmt(fmt) "g_ffs: " fmt
+
 #include <linux/module.h>
 #include <linux/utsname.h>