edac: i7300_edac: Fix 'may be used uninitialized' warning

gcc 4.7 warns that nr_pages may be used uninitialized in
i7300_init_csrows().  In fact, the case where it's not initialised is
an error that will result in returning early without using it.
Silence the warning by initialising to 0.

This was done upstream as part of commit 084a4fccef39 "edac: move dimm
properties to struct dimm_info".

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>