ath79: fix IMAGE_SIZE for common TP-Link definitions

ath79: fix IMAGE_SIZE for common TP-Link definitions

So far, IMAGE_SIZE is set as follows:
tplink-4m*   3904k  0x3d0000
tplink-8m*   7936k  0x7c0000
tplink-16m* 15872k  0xf80000

However, based on the size of firmware partitions in DTS it should
be:
tplink-4m*   3904k  0x3d0000
tplink-8m*   8000k  0x7d0000
tplink-16m* 16192k  0xfd0000

All (!) 8m*/16m* devices actually follow the latter scheme, which
is also consistent in terms of left free space for other
partitions. Thus, fix it.

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