fel: Rework some of the USB functions' logic

This moves claiming / releasing the interface into the respective
"open" / "close" functions. The USB code in main() is now trimmed
down to:

feldev_init();
handle = open_fel_device(...);
feldev_done(handle);
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>

fel.c

@@ -1422,11 +1422,8 @@ static feldev_handle *open_fel_device(int busnum, int devnum,
 			}
 			exit(1);
 		}
-		return result;
-	}
-
+	} else {
 		/* look for specific bus and device number */
-	pr_info("Selecting USB Bus %03d Device %03d\n", busnum, devnum);
 		bool found = false;
 		ssize_t rc, i;
 		libusb_device **list;
@@ -1461,13 +1458,36 @@ static feldev_handle *open_fel_device(int busnum, int devnum,
 				busnum, devnum);
 			exit(1);
 		}
+	}
+
+	feldev_claim(result); /* claim interface, detect USB endpoints */
+
 	return result;
 }
 
 void feldev_close(feldev_handle *dev)
 {
+	if (dev) {
+		if (dev->usb->handle) {
+			feldev_release(dev);
 			libusb_close(dev->usb->handle);
-	free(dev->usb); /* release memory allocated for felusb_handle struct */
+		}
+		free(dev->usb); /* release memory allocated for felusb_handle */
+	}
+}
+
+void feldev_init(void)
+{
+	int rc = libusb_init(NULL);
+	if (rc != 0)
+		usb_error(rc, "libusb_init()", 1);
+}
+
+void feldev_done(feldev_handle *dev)
+{
+	feldev_close(dev);
+	free(dev);
+	libusb_exit(NULL);
 }
 
 int main(int argc, char **argv)
@@ -1539,17 +1559,15 @@ int main(int argc, char **argv)
 				fprintf(stderr, "ERROR: Expected 'bus:devnum', got '%s'.\n", dev_arg);
 				exit(1);
 			}
+			pr_info("Selecting USB Bus %03d Device %03d\n", busnum, devnum);
 		} else
 			break; /* no valid (prefix) option detected, exit loop */
 		argc -= 1;
 		argv += 1;
 	}
 
-	int rc = libusb_init(NULL);
-	assert(rc == 0);
+	feldev_init();
 	handle = open_fel_device(busnum, devnum, AW_USB_VENDOR_ID, AW_USB_PRODUCT_ID);
-	assert(handle != NULL);
-	feldev_claim(handle);
 
 	while (argc > 1 ) {
 		int skip = 1;
@@ -1645,10 +1663,7 @@ int main(int argc, char **argv)
 		aw_fel_execute(handle, uboot_entry);
 	}
 
-	feldev_release(handle);
-	feldev_close(handle);
-	free(handle);
-	libusb_exit(NULL);
+	feldev_done(handle);
 
 	return 0;
 }