usb: gadget: fsl_udc: fix dequeuing a request in progress

The original implementation of dequeuing a request in progress
is not correct.  Change to use a correct process and also clean
up the related functions a little bit.

Signed-off-by: Li Yang <leoli@freescale.com>
Acked-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>