bus: fsl-mc: fix use-after-free in driver_override_show()

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

bus: fsl-mc: fix use-after-free in driver_override_show()

The driver_override_show() function reads the driver_override string

without holding the device_lock. However, driver_override_store() uses

driver_set_override(), which modifies and frees the string while holding

the device_lock.

This can result in a concurrent use-after-free if the string is freed

by the store function while being read by the show function.

Fix this by holding the device_lock around the read operation.