um: ubd: Retry buffer read on any kind of error

Should bulk_req_safe_read return an error, we want to retry the read,
otherwise, even though no IO will be done, os_write_file might still end
up writing garbage to the pipe.

Cc: Martyn Welch <martyn.welch@collabora.com>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Richard Weinberger <richard@nod.at>