i7300_edac: pre-allocate a buffer used to prepare err messages

Instead of dynamically allocating a buffer for it where needed,
just allocate it once. As we'll use the same buffer also during
fatal and non-fatal errors, is is very risky to dynamically allocate
it during an error.

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