fpga: fpga-mgr: fix kernel-doc warnings

warnings: No description found for return value of 'xxx'

In-order to fix the above kernel-doc warnings added the
'Return' description for 'devm_fpga_mgr_register_full()'
and 'devm_fpga_mgr_register()' APIs.
Signed-off-by: Nava kishore Manne <nava.manne@xilinx.com>
Link: https://lore.kernel.org/r/20220423170235.2115479-4-nava.manne@xilinx.comSigned-off-by: Xu Yilun <yilun.xu@intel.com>

drivers/fpga/fpga-mgr.c

@@ -731,6 +731,8 @@ static void devm_fpga_mgr_unregister(struct device *dev, void *res)
  * @parent:	fpga manager device from pdev
  * @info:	parameters for fpga manager
  *
+ * Return:  fpga manager pointer on success, negative error code otherwise.
+ *
  * This is the devres variant of fpga_mgr_register_full() for which the unregister
  * function will be called automatically when the managing device is detached.
  */
@@ -764,6 +766,8 @@ EXPORT_SYMBOL_GPL(devm_fpga_mgr_register_full);
  * @mops:	pointer to structure of fpga manager ops
  * @priv:	fpga manager private data
  *
+ * Return:  fpga manager pointer on success, negative error code otherwise.
+ *
  * This is the devres variant of fpga_mgr_register() for which the
  * unregister function will be called automatically when the managing
  * device is detached.