xilinx: zynqmp: Handle fallthrough statement properly

Now intentional fallthrough in switch-case should be labelled that's why
convert comment.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>
Link: https://lore.kernel.org/r/b3e976bd3b9bbe7061fec5ba0a3b4e78fdd78394.1634556602.git.michal.simek@xilinx.com