net: usb: qmi_wwan: restore mtu min/max values after raw_ip switch

usbnet creates network interfaces with min_mtu = 0 and
max_mtu = ETH_MAX_MTU.

These values are not modified by qmi_wwan when the network interface
is created initially, allowing, for example, to set mtu greater than 1500.

When a raw_ip switch is done (raw_ip set to 'Y', then set to 'N') the mtu
values for the network interface are set through ether_setup, with
min_mtu = ETH_MIN_MTU and max_mtu = ETH_DATA_LEN, not allowing anymore to
set mtu greater than 1500 (error: mtu greater than device maximum).

The patch restores the original min/max mtu values set by usbnet after a
raw_ip switch.

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