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>