ar71xx: Fix IMAGE_SIZE for TP-Link Archer C7 v5

ar71xx: Fix IMAGE_SIZE for TP-Link Archer C7 v5

IMAGE_SIZE for C7v5 is wrong in openwrt-18.06, looks like it
was just copied from C7v4. In master, this got fixed with the
introduction of dynamic partitioning in
https://github.com/openwrt/openwrt/commit/7c78be1b747eb0c8d64da67deb3a8aec75bd7b9c

However, this is not connected to the changes introduced there,
but also applies to the static partitioning in openwrt-18.06.
It appears to be simply wrong at the moment ...

Tested-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>