greybus: loopback: add missing pr_fmt

Add missing pr_fmt so we can at least tell what module the sole
remaining pr_err was from.

Testing Done:
Tested on DB3.5 with the generic bridge firmware on APB2.
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/loopback.c

@@ -6,6 +6,9 @@
  *
  * Released under the GPLv2 only.
  */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/mutex.h>