net: dsa: microchip: Added the condition for scheduling ksz_mib_read_work

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

net: dsa: microchip: Added the condition for scheduling ksz_mib_read_work

When the ksz module is installed and removed using rmmod, kernel crashes

with null pointer dereferrence error. During rmmod, ksz_switch_remove

function tries to cancel the mib_read_workqueue using

cancel_delayed_work_sync routine and unregister switch from dsa.

During dsa_unregister_switch it calls ksz_mac_link_down, which in turn

reschedules the workqueue since mib_interval is non-zero.

Due to which queue executed after mib_interval and it tries to access

dp->slave. But the slave is unregistered in the ksz_switch_remove

function. Hence kernel crashes.

To avoid this crash, before canceling the workqueue, resetted the

mib_interval to 0.

v1 -> v2:

-Removed the if condition in ksz_mib_read_work