i7300_edac: Properly initialize per-csrow memory size

Due to the current edac-core limits, we cannot represent a per-channel
memory size, for FB-DIMM drivers. So, we need to sum-up all values
for each slot, in order to properly represent the total amount of
memory found by the i7300 driver.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>