net: cdc_ether: Ensure that default MTU is minimum 2048

The default MTU size for a usbnet netdev is 1514. When the VLAN
packets of 1468 < size <= 1472 are sent from connected device,
they come without fragmentation as a single chunk of size greater
than 1514, resulting in the transactions failing with EOVERFLOW
error. Fix this by ensuring the minimum default MTU size for CDC
ethernet devices is 2k.

Change-Id: If4001bafc329b29dc1c90cef0897309ed4e0febd
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>