usb: gadget: Send DSR linestate on DTR high from host

When the host PC sends a DTR high during DUN call,
it expects to get a DSR signal from the modem in reply.
Add necessary support for that.

Change-Id: If46c4b75fa6210c2b15cd2880bc93461fa3a4e89
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>