pieusb: Fix [-Wmisleading-indentation] compiler warning

The indentation has been aligned what is used inside the while loop.
The function appears to have been copied from elsewhere and slightly
modified.

The msg variable is set to NULL inside the if branch for efficiency
only.

backend/pieusb_usb.c

@@ -179,8 +179,10 @@ _hexdump(char *msg, unsigned char *ptr, int size)
     while (size-- > 0)
     {
 	if ((count % 16) == 0)
+	{
 	    fprintf (stderr, "%s\t%08lx:", msg?msg:"", start);
 	    msg = NULL;
+	}
 	fprintf (stderr, " %02x", *ptr++);
 	count++;
 	start++;