firmware: xilinx: Fix kernel-doc warnings

kernel-doc is reporting some style issues. The patch is fixing them.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/e606233d15bfdc594535dd34eb85472b42f61830.1606832997.git.michal.simek@xilinx.com