apm821xx: Netgear WNDR4700 limit kernel lzma dictionary

apm821xx: Netgear WNDR4700 limit kernel lzma dictionary

WNDR4700 uboot has an issue with decompressing kernel with default dictionary size (-d23 which is about 8MB).
Limiting lzma dictionary lowers memory footprint and allows device to boot the kernel.

The highest bootable dictonary size is 18, choosing 16 for an extra safety margin.

Kernel size befor and after:
-d23: 2663665 Bytes
-d16: 2892757 Bytes

Kernel size increased by 230kB (9%)

Fixes: FS#3258
Signed-off-by: Wiktor Stasiak <wiktor.stasiak@gmail.com>