EDAC: Remove EDAC_DIMM_OFF() macro

The EDAC_DIMM_OFF() macro takes 5 arguments to get the DIMM's index.
Simplify this by storing the index in struct dimm_info to avoid its
calculation and remove the EDAC_DIMM_OFF() macro. The index can be
directly used then.

Another advantage is that edac_mc_alloc() could be used even if the
exact size of the layers is unknown. Only the number of DIMMs would be
needed.

Rename iterator variable to idx, while at it. The name is more handy,
esp. when searching for it in the code.

Signed-off-by: Robert Richter <rrichter@marvell.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: "linux-edac@vger.kernel.org" <linux-edac@vger.kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20191106093239.25517-3-rrichter@marvell.com