USB: gadget: u_bam: track dl packets pending for the host

Add a counter to track dl packets pending for the host. This counter
along with tx_skb_q.qlen tells how many dl packets are pending with
USB at any point and helpful in debugging data stall issues. This
counter value can be read from /sys/kernel/debug/usb_rmnet/status
file.

Change-Id: I9c0104611be6881e431ade9251f72876c32cfab4
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>