usb: serial: csvt: Fix TIOCMSET ioctl implementation

The current code is incorrectly converting TTY DTR/RTS
signals to CDC ACM bit definitions.  Due to this DTR
is never dropped when DUN call is disconnected.

CRs-Fixed: 478719
Change-Id: If45a09ff201252dd891b92a85420726a9de49dc6
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>