qmi_wwan: add MTU default to qmap network interface

This patch adds MTU default value to qmap network interface in
order to avoid "RTNETLINK answers: No buffer space available"
error when setting an ipv6 address.

Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Acked-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>