ppp: fix keep alive option

currently the keepalive option needs to be removed to fully disable it. this patch allows us to set it to 0.

Signed-off-by: John Crispin <blogic@openwrt.org>

SVN-Revision: 41438

package/network/services/ppp/files/ppp.sh

@@ -32,7 +32,7 @@ ppp_generic_setup() {
 	else
 		demand="persist"
 	fi
-
+	[ "${keepalive:-0}" -lt 1 ] && keepalive=""
 	[ -n "$mtu" ] || json_get_var mtu mtu
 	[ -n "$pppname" ] || pppname="${proto:-ppp}-$config"