usb: gadget: f_uvc: Store EP0 control request state during setup stage

To handle class requests received on ep0, the driver needs to access the
length and direction of the request after the setup stage. It currently
stores them in a v4l2 event during the setup stage, and then copies them
from the event structure to the driver internal state structure when the
event is dequeued.

This two-steps approach isn't necessary. Simplify the driver by storing
the needed information in the driver internal state structure directly
during the setup stage.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>