Fix route setup for PPP links

The `mptcp_up` script did not correctly set up the routes for my 4G/LTE USB Modems. I found the following in `/etc/network/if_up_env`:
``` bash
METHOD=NetworkManager
MODE=start
CONNECTION_UUID=ff473cf6-44dc-411e-addf-b598975d097e
LOGICAL=ppp0
DEVICE_IFACE=ttyUSB1
IP4_NAMESERVERS=10.8.11.21 10.8.11.22
PHASE=post-up
ADDRFAM=NetworkManager
IP4_NUM_ADDRESSES=1
VERBOSITY=0
IP4_NUM_ROUTES=0
IP4_ADDRESS_0=10.160.168.121/32 10.64.64.64
DEVICE_IP_IFACE=ppp0
IFACE=ppp0
CONNECTION_ID=Orange
PWD=/
```
The command executed by the script is therefore:
```
ip route add table ttyUSB1 default dev ttyUSB1 scope link
```
which gives me: Cannot find device "ttyUSB1"

This change fixed the problem for me.