i7core_edac: fix panic in udimm sysfs attributes registration

Array of udimm sysfs attributes was not ended with NULL marker, leading to
dereference of random memory.

  EDAC DEBUG: edac_create_mci_instance_attributes: edac_create_mci_instance_attributes() file udimm0
  EDAC DEBUG: edac_create_mci_instance_attributes: edac_create_mci_instance_attributes() file udimm1
  EDAC DEBUG: edac_create_mci_instance_attributes: edac_create_mci_instance_attributes() file udimm2
  BUG: unable to handle kernel NULL pointer dereference at 00000000000001a4
  IP: [<ffffffff81330b36>] edac_create_mci_instance_attributes+0x148/0x1f1
  Pid: 1, comm: swapper Not tainted 2.6.36-rc3-nv+ #483 P6T SE/System Product Name
  RIP: 0010:[<ffffffff81330b36>]  [<ffffffff81330b36>] edac_create_mci_instance_attributes+0x148/0x1f1
  (...)
  Call Trace:
   [<ffffffff81330b86>] edac_create_mci_instance_attributes+0x198/0x1f1
   [<ffffffff81330c9a>] edac_create_sysfs_mci_device+0xbb/0x2b2
   [<ffffffff8132f533>] edac_mc_add_mc+0x46b/0x557
   ...