PC300 MLPPP support is currently marked broken in 2.6.x.

To fix that, attached patch implements tiocmset/tiocmget methods on the
pc300_tty.c driver, which is the new method tty drivers are supposed to
use instead ioctl.

This fixes two related issues in the ioctl handler:

- ioctl requesting RTS signal would affect DTR signal
- The RTS signal is now handled.

Bonus: Throw out unused ioctl handler