dropbear: fix keepalive with putty

Don't send SSH_MSG_UNIMPLEMENTED for keepalive responses, which broke
at least putty.

Fixes #17522 / #17523.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42162 3c298f89-4303-0410-b956-a3cf2f4a3e73