EDAC/igen6: Avoid segmentation fault on module unload

The segmentation fault happens because:

During modprobe:
1. In igen6_probe(), igen6_pvt will be allocated with kzalloc()
2. In igen6_register_mci(), mci->pvt_info will point to
   &igen6_pvt->imc[mc]

During rmmod:
1. In mci_release() in edac_mc.c, it will kfree(mci->pvt_info)
2. In igen6_remove(), it will kfree(igen6_pvt);

Fix this issue by setting mci->pvt_info to NULL to avoid the double
kfree.

Fixes: 10590a9d ("EDAC/igen6: Add EDAC driver for Intel client SoCs using IBECC")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219360


Signed-off-by: Orange Kao <orange@aiven.io>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20241104124237.124109-2-orange@aiven.io