fetch2: Fix missing function call

Fix the issue:

File: '/srv/home/pokybuild/yocto-autobuilder-new/yocto-slave/build-appliance/build/bitbake/lib/bb/fetch2/__init__.py', lineno: 813, function: try_mirror_url
     0809:    except bb.fetch2.BBFetchException as e:
     0810:        if isinstance(e, ChecksumError):
     0811:            logger.warn("Mirror checksum failure for url %s (original url: %s)\nCleaning and trying again." % (newuri, origud.url))
     0812:            logger.warn(str(e))
 *** 0813:            self.rename_bad_checksum(ud, e.checksum)
     0814:        elif isinstance(e, NoChecksumError):
     0815:            raise
     0816:        else:
     0817:            logger.debug(1, "Mirror fetch failure for url %s (original url: %s)" % (newuri, origud.url))
Exception: NameError: global name 'self' is not defined

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>