EDAC/igen6: Fix NULL pointer dereference

EDAC/igen6: Fix NULL pointer dereference

A kernel panic was reported with the following kernel log:

  EDAC igen6: Expected 2 mcs, but only 1 detected.
  BUG: unable to handle page fault for address: 000000000000d570
  ...
  Hardware name: Notebook V54x_6x_TU/V54x_6x_TU, BIOS Dasharo (coreboot+UEFI) v0.9.0 07/17/2024
  RIP: e030:ecclog_handler+0x7e/0xf0 [igen6_edac]
  ...
  igen6_probe+0x2a0/0x343 [igen6_edac]
  ...
  igen6_init+0xc5/0xff0 [igen6_edac]
  ...

This issue occurred because one memory controller was disabled by
the BIOS but the igen6_edac driver still checked all the memory
controllers, including this absent one, to identify the source of
the error. Accessing the null MMIO for the absent memory controller
resulted in the oops above.

Fix this issue by reverting the configuration structure to non-const
and updating the field 'res_cfg->num_imc' to reflect the number of
detected memory controllers.

Fixes: 20e190b1c1fd ("EDAC/igen6: Skip absent memory controllers")
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Closes: https://lore.kernel.org/all/aFFN7RlXkaK_loQb@mail-itl/
Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Link: https://lore.kernel.org/r/20250618162307.1523736-1-qiuxu.zhuo@intel.com