bus: fsl-mc: fix double-free on mc_dev

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

bus: fsl-mc: fix double-free on mc_dev

The blamed commit tried to simplify how the deallocations are done but,

in the process, introduced a double-free on the mc_dev variable.

In case the MC device is a DPRC, a new mc_bus is allocated and the

mc_dev variable is just a reference to one of its fields. In this

circumstance, on the error path only the mc_bus should be freed.

This commit introduces back the following checkpatch warning which is a

false-positive.

WARNING: kfree(NULL) is safe and this check is probably not required

+ if (mc_bus)

+ kfree(mc_bus);