net: usb: smsc75xx: fix mtu

Make smsc75xx recalculate the hard_mtu after adjusting the
hard_header_len.

Without this, usbnet adjusts the MTU down to 1492 bytes, and the host is
unable to receive standard 1500-byte frames from the device.

Inspired by same fix on cdc_eem 78fb72f7.

Tested on ARM/Omap3 with EVB-LAN7500-LC.
Signed-off-by: Stephane Fillod <fillods@users.sf.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/smsc75xx.c

@@ -1051,6 +1051,7 @@ static int smsc75xx_bind(struct usbnet *dev, struct usb_interface *intf)
 	dev->net->ethtool_ops = &smsc75xx_ethtool_ops;
 	dev->net->flags |= IFF_MULTICAST;
 	dev->net->hard_header_len += SMSC75XX_TX_OVERHEAD;
+	dev->hard_mtu = dev->net->mtu + dev->net->hard_header_len;
 	return 0;
 }