ramips: use full 8MB flash on ZyXEL Keenetic

ramips: use full 8MB flash on ZyXEL Keenetic

ZyXEL Keenetic has 8MB flash, but OpenWrt uses only 4MB.
This commit fixes the problem.

WikiDevi page [1] says that ZyXEL Keenetic has FLA1: 8 MiB, there is
an article with specs [2] (in Russian).

[1] https://wikidevi.wi-cat.ru/ZyXEL_Keenetic
[2] https://3dnews.ru/608774/page-2.html

Fixes: FS#2487
Fixes: a7cbf59e0e04 ("ramips: add new device ZyXEL Keenetic as kn")

Signed-off-by: Alexey Dobrovolsky <dobrovolskiy.alexey@gmail.com>
(cherry picked from commit fea232ae8feb6af780fd4fa78ebe9231778bf75a)