usb: gadget: u_ether: Don't drop IP packets with size > ETH_FRAME

Driver drops RX packets if size is greater than ETH max packet size
i.e. 1500. But this can happen in IP mode, hence don't drop such
large RX packets in IP mode.

Change-Id: I9e916dc85ed8dea3d3d45ae8756485c14e3b2ac0
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>