edac: Only expose csrows/channels on legacy API if they're populated

This patch actually fixes a bug with the legacy API, where, at the
same csrow, some channels may have different DIMMs. This can happen
on FB-DIMM/RAMBUS and modern Intel controllers.

This is the case, for example, of Nehalem machines:

$ ./edac-ctl --layout
       +-----------------------------------+
       |                mc0                |
       | channel0  | channel1  | channel2  |
-------+-----------------------------------+
slot2: |     0 MB  |     0 MB  |     0 MB  |
slot1: |  1024 MB  |     0 MB  |     0 MB  |
slot0: |  1024 MB  |  1024 MB  |  1024 MB  |
-------+-----------------------------------+

Before this patch, non-filled memories were shown. Now, only what's
filled is there:

grep . /sys/devices/system/edac/mc/mc0/csrow*/ch?*
/sys/devices/system/edac/mc/mc0/csrow0/ch0_ce_count:0
/sys/devices/system/edac/mc/mc0/csrow0/ch0_dimm_label:CPU#0Channel#0_DIMM#0
/sys/devices/system/edac/mc/mc0/csrow0/ch1_ce_count:0
/sys/devices/system/edac/mc/mc0/csrow0/ch1_dimm_label:CPU#0Channel#0_DIMM#1
/sys/devices/system/edac/mc/mc0/csrow1/ch0_ce_count:0
/sys/devices/system/edac/mc/mc0/csrow1/ch0_dimm_label:CPU#0Channel#1_DIMM#0
/sys/devices/system/edac/mc/mc0/csrow2/ch0_ce_count:0
/sys/devices/system/edac/mc/mc0/csrow2/ch0_dimm_label:CPU#0Channel#2_DIMM#0

Thanks-to: Aristeu Rozanski Filho <arozansk@redhat.com>
Reviewed-by: Aristeu Rozanski <arozansk@redhat.com>
Cc: Doug Thompson <norsk5@yahoo.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>