fix ppp unit number issues (#2178)

SVN-Revision: 8665

package/ppp/files/ppp.sh

 scan_ppp() {
 	config_get ifname "$1" ifname
 	pppdev="${pppdev:-0}"
-	config_set "$1" ifname "ppp$pppdev"
-	config_set "$1" unit "$pppdev"
-	pppdev="$(($pppdev + 1))"
+	config_get unit "$1" unit
+	[ -z "$unit" ] && {
+		config_set "$1" ifname "ppp$pppdev"
+		config_set "$1" unit "$pppdev"
+		pppdev="$(($pppdev + 1))"
+	}
 }
 
 start_pppd() {