net: rmnet_usb: Set default MTU to 2048 bytes

The default MTU size for a usbnet netdev is 1514. When connected
device wants to send large chunk of data, the xHC will see Babble
Detected Error (TRB completion code 3) because of incoming packet
size greater than 490 (1514 - 1024).
Fix this by setting default MTU for rmnet_usb devices to 2048.
It is expected that netmgr will set it to 16k for actual usecase.

Change-Id: I3855276cc22fdc016fca3188e75376e990074792
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>