bcm47xx: disable image for Linksys WRT160n v3
The device can only hold 3.6 MB, but newer images (since 18.06)
are bigger, so flashing a new version fails.
This disables default build for this device based on the bug report
referenced below.
Fixes: FS#1963
Signed-off-by: Adrian Schmutzler <[email protected]>