spi: spi-zynqmp-gqspi: Fix kernel-doc warnings

Fix kernel-doc warnings in ZynqMP qspi driver file.

Signed-off-by: Amit Kumar Mahapatra <amit.kumar-mahapatra@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/ba5920c57eee06fafa6f9d1df9859e69819ac301.1600931476.git.michal.simek@xilinx.com
Signed-off-by: Mark Brown <broonie@kernel.org>