usb: gadget: ether: return error from rx_submit if no request

This prevents a crash if tftpboot is given a bad filename.

rx_req will be released by eth_reset_config
which is called by eth_disconnect,
which is called using the .disconnect member of usb_gadget_driver by mv_pullup in mv_udc
which is called using the .pullup member of usb_gadget_ops by usb_gadget_disconnect
which is called by usb_eth_halt
which is called using the .halt member of eth_device by eth_halt
which is called by TftpHandler when TFTP_ERR_FILE_NOT_FOUND or TFTP_ERR_ACCESS_DENIED occurs

I trigger this with the following commands
setenv ipaddr 10.0.0.2 && setenv netmask 255.255.255.0 && setenv serverip 10.0.0.1
setenv usbnet_devaddr 00:11:22:33:44:55 && setenv usbnet_hostaddr 00:aa:bb:cc:dd:ee
setenv ethprime usb_ether && setenv ethact usb_ether && setenv ncip 10.0.0.1
tftpboot 10800000 10.0.0.1:missing_file

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>