USB: Gadget: u_serial: Debugfs for endpoint buffer monitoring

The debugfs used in u_serial driver provides the information of the number
of bytes transferred between host and tty and dtr flag status using
the readstatus command.

Change-Id: I761aaedf1693944b8c7bf7e207e1f271d1b48a98
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>