xilinx: Fix logic when dfu_alt_info is generated

Generate dfu_alt_info only when it is not defined.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/464e4b325c644e52a660df9cf44eeb4d80427f6a.1660055571.git.michal.simek@amd.com