ramips: only limit lzma dictionary size on mt7621

ramips: only limit lzma dictionary size on mt7621

The changed dictionary size leads to a different LZMA header which breaks
sysupgrade image magic checkibng on at least some RT288x boards.

Since the commit message only mentions testing on MT7621 and since the
change appears to break at least one other ramips subtarget, do not take
any chances and restrict the size limitation to only MT7621.

Fixes FS#1797
Fixes 09b6755946 ("ramips: limit dictionary size for lzma compression")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>