usb: gadget: f_diag: Decrement counter if zero length packet queueing fails

If usb_ep_queue() fails on queueing zero-length packet, driver is not
decrementing dpkts_tolaptop_pending counter which may results into seeing
count mismatch. Hence decrement dpkts_tolaptop_pending if zero length
packet queueing fails.

CRs-Fixed: 1027031
Change-Id: Id3c7c2627bdf37524067512db51d3180c570106d
Signed-off-by: Mayank Rana <mrana@codeaurora.org>