USB: ssu100: switch to generic TIOCMIWAIT implementation

Switch to the generic TIOCMIWAIT implementation.

This also fixes the issue with processes waiting for
modem-status-changes not being woken up at disconnect.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>