usb: gadget: g_ether: fix frame size check

Checking skb->len against ETH_FRAME_LEN assumes a 1514
ethernet frame size. With an 802.1Q VLAN header, ethernet
frame length can now be 1518. Validate frame length against that.

Signed-off-by: Ian Coolidge <iancoolidge@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>