staging: octeon: add missing napi_disable in cvm_oct_rx_shutdown

In the Linux kernel, the following vulnerability has been resolved:

staging: octeon: add missing napi_disable in cvm_oct_rx_shutdown

cvm_oct_rx_shutdown calls free_irq and netif_napi_del without
disabling the napi instance first. As the free_irq only waits
for completion of hard interrupt handlers, the napi poll
function could still be active. If cvm_oct_remove proceeds to
free the plat structure (which holds the NAPI instances), the
active poll function will access freed memory, resulting in a
use-after-free crash.