USB-CDC: Prevent rx_req being enqueued twice

USB-CDC: Prevent rx_req being enqueued twice

After gadget reinitializaton (after tftp has been done once)
packet_received may become equal to 1 due to nuking OUT_EP
while disabling it in eth_reset_config.

rx_submit called from usb_eth_init queues rx_req first time.
But the first call of usb_eth_recv from NetLoop queues rx_req
again due to packet_received = 1.

The following flow shows the path of functions calls when
this happens:

 net/net.c:NetLoop
 |
 +-net/eth.c:eth_init
 |   ether.c:usb_eth_init
 |   |
 |   +-udc_driver:usb_gadget_handle_interrupts
 |   |   udc_driver:...
 |   |     ether.c:eth_setup
 |   |       ether.c:eth_set_config
 |   |         ether.c:eth_reset_config
 |   |           udc_driver:usb_ep_disable
 |   |             udc_driver:nuke
 |   |               ether.c:rx_complete
 |   |                 ether.c: packet_received = 1;
 |   |
 |   +-ether.c:rx_submit
 |       udc_driver:usb_ep_queue --- The first time when rx_req is queued
 |
 +-net/eth.c:eth_rx
     ether.c:usb_eth_recv
     |
     +-udc_driver:usb_gadget_handle_interrupts
     |   udc_driver:... --- no interrupts, returning
     +-ether.c: if (packet_received) { ...
         ether.c:rx_submit
           udc_driver:usb_ep_queue --- The second time!

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