powerpc/powernv: Report size of OPAL memcons log

The OPAL memory console is reported to be size zero, as we do not
initialise the struct attr with any size information due to the size
being variable. This leads users to think that the console is empty.

Instead report the maximum size.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>