staging: dgcn: removes unnecessary commands in ioctl

The commands TIOCGETP, TCGETS, and TCGETA are not supposed to be seen by
the ioctl. This patch removes the switch cases for these commands.

Signed-off-by: Lidza Louina <lidza.louina@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>