Modified do_upload() to use UPLOAD_ERR constants.

Modified switchcase in the do_upload() use the UPLOAD_ERR_* constants,
instead of just using an integer, and then commenting out the constant
beside it.
diff --git a/system/libraries/Upload.php b/system/libraries/Upload.php
index 1c14f99..0c6b7e6 100644
--- a/system/libraries/Upload.php
+++ b/system/libraries/Upload.php
@@ -366,25 +366,25 @@
 
 			switch ($error)
 			{
-				case 1:	// UPLOAD_ERR_INI_SIZE
+				case UPLOAD_ERR_INI_SIZE:
 					$this->set_error('upload_file_exceeds_limit');
 					break;
-				case 2: // UPLOAD_ERR_FORM_SIZE
+				case UPLOAD_ERR_FORM_SIZE:
 					$this->set_error('upload_file_exceeds_form_limit');
 					break;
-				case 3: // UPLOAD_ERR_PARTIAL
+				case UPLOAD_ERR_PARTIAL:
 					$this->set_error('upload_file_partial');
 					break;
-				case 4: // UPLOAD_ERR_NO_FILE
+				case UPLOAD_ERR_NO_FILE:
 					$this->set_error('upload_no_file_selected');
 					break;
-				case 6: // UPLOAD_ERR_NO_TMP_DIR
+				case UPLOAD_ERR_NO_TMP_DIR:
 					$this->set_error('upload_no_temp_directory');
 					break;
-				case 7: // UPLOAD_ERR_CANT_WRITE
+				case UPLOAD_ERR_CANT_WRITE:
 					$this->set_error('upload_unable_to_write_file');
 					break;
-				case 8: // UPLOAD_ERR_EXTENSION
+				case UPLOAD_ERR_EXTENSION:
 					$this->set_error('upload_stopped_by_extension');
 					break;
 				default: