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

commit  452a99068fe59a6299ac059e7c2ff354e87a32d7 from
https://github.com/nxp-imx/linux-imx lf-6.1.y

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);

Fixes: a042fbed

 ("staging: fsl-mc: simplify couple of deallocations")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>