usb: gadget: u_bam: Enable zero length packet for tx transfers

Avoid packet re-transmission/drop by sending zero length packet
when usb transfer is multiple of usb endpoint max packet size.

CRs-Fixed: 494184
Change-Id: I01c9f9be41d3b64760e9bc08ec82ef8b6cc1a80f
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>