USB: QTI: Add counters for no. of packets sent over qti control xport

Add counters to track the number of packets sent to host, to modem from
qti control driver and also number of packets dropped by modem. Add a new
debugfs entry. The stats can be read from
    /d/usb_qti/status.

Change-Id: I8938d2e44eea0fc9eb8a090acee9ee3b0e10f27a
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>
Signed-off-by: Tarun Gupta <tarung@codeaurora.org>