usb: gadget: uvc: Merge the streaming maxpacket and mult parameters

Compute the multiplier from the maximum packet size based on the speed.

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>