usb: gadget: uvc: Rename STATUS_BYTECOUNT to UVC_STATUS_MAX_PACKET_SIZE

Descriptive names make the code more readable.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Bhupesh Sharma <bhupesh.sharma@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>