tty/hvc_iucv: Disconnect IUCV connection when lowering DTR

Implement the dtr_rts() hvc console callback to improve control when to
disconnect the IUCV connection.  Previously, the IUCV connection was
disconnected during the notifier_del() callback, i.e., when the last file
descriptor to the hvc terminal device was closed.

Recent changes in login programs caused undesired disconnects during the
login phase.  To prevent these kind of disconnects, implement the dtr_rts
callback to implicitly handle the HUPCL termios control via the hvc_console
driver.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>