i7300_edac: Fix error flag testing

* Right-shift the values in GET_FBD_FAT_IDX and GET_FBD_NF_IDX, so
  that the callers get the result they expect.
* Fix definition of FERR_FAT_FBD_ERR_MASK.
* Call GET_FBD_NF_IDX, not GET_FBD_FAT_IDX, when operating on
  register FERR_NF_FBD. We were lucky they have the same definition.

This fixes kernel bug #44131:
https://bugzilla.kernel.org/show_bug.cgi?id=44131

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Mauro Carvalho Chehab <mchehab@redhat.com>
Cc: Doug Thompson <dougthompson@xmission.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>