lib/flash: Update type of size field

Skiboot commit c043065 "flash: Make size 64 bit safe" updated the
prototype of blocklevel_get_info() to use a uint64_t for the size
parameter. Update our usage to reflect this.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
(cherry picked from commit 48ca02faa9816b158cb6fa56a4107701143cb212)