Implement endpoint dequeue callback function.

Without this function, uboot will hang when executing fastboot comamnd.
See following flow:
"fastboot_tx_write_str->fastboot_tx_write->usb_ep_dequeue->ep->ops->dequeue"
without implement ci_udc dequeue function, ep->ops->dequeue is NULL, then
uboot will hang.

Tested on mx6qsabresd board with fastboot enabled.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Cc: "Łukasz Majewski" <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>