usb: cdns3: gadget: clear trb->length as zero after preparing every trb

It clears trb->length as zero before preparing td, but if scatter
buffer is used for td, there are several trbs within td, it needs to clear
every trb->length as zero, otherwise, the default value for trb->length
may not be zero after it begins to use the second round of trb rings.

Fixes: abc6b579 ("usb: cdns3: gadget: using correct sg operations")
Signed-off-by: Peter Chen <peter.chen@nxp.com>