usb: cdns3: Variable 'length' set but not used

Patch removes not used variable 'length' from
cdns3_wa2_descmiss_copy_data function.

Fixes: 141e70fe ("usb: cdns3: gadget: need to handle sg case for workaround 2 case")
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Pawel Laszczak <pawell@cadence.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>