USB-CDC: Fix tx/rx_req memory leaks

USB-CDC: Fix tx/rx_req memory leaks

Remove and fix needless and destructive operations with tx/rx_req.

1) 'req' in rx_complete is always not NULL and always equals to rx_req
2) Free allocated tx_req if rx_req allocation has failed
3) Do not zero out tx/rx_req in usb_eth_init, leave this for
eth_reset_config which will be called at the next use of usb0
4) Additional check in usb_eth_recv is not required

Signed-off-by: Vitaly Kuzmichev <vkuzmichev@mvista.com>