usb: serial: Enable dynamic transport type switch for DUN calls

Transport type can now be switched from TTY to SMD using an
IOCTL command.
When the DUN call ends, the transport type changes back to TTY.
This feature is required for running SoftAP concurrently with DUN calls.

Change-Id: I42262bb832982a3fc8ddb2f1063250a99e4b4c69
Signed-off-by: Danny Segal <dsegal@codeaurora.org>
Signed-off-by: Lena Salman <esalman@codeaurora.org>