net: usb: cdc_eem: fix mtu

Make CDC EEM recalculate the hard_mtu after adjusting the
hard_header_len.

Without this, usbnet adjusts the MTU down to 1494 bytes, and the host is
unable to receive standard 1500-byte frames from the device.

Tested with the Linux USB Ethernet gadget.

Cc: Oliver Neukum <oliver@neukum.name>
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/cdc_eem.c

@@ -93,6 +93,7 @@ static int eem_bind(struct usbnet *dev, struct usb_interface *intf)
 	/* no jumbogram (16K) support for now */
 
 	dev->net->hard_header_len += EEM_HEAD + ETH_FCS_LEN;
+	dev->hard_mtu = dev->net->mtu + dev->net->hard_header_len;
 
 	return 0;
 }