powerpc/powernv: Pass buffer size to OPAL validate flash call

We pass actual buffer size to opal_validate_flash() OPAL API call
and in return it contains output buffer size.

Commit cc146d1d (Fix little endian issues) missed to set the size
param before making OPAL call. So firmware image validation fails.

This patch sets size variable before making OPAL call.

Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Tested-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>