staging: gdm724x: constify tty_port_operations structs

Constifies tty_port_operations structure in
the tty code of the gdm724x driver since it
is not modified after its initialization.

Detected and found using Coccinelle.

Suggested-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Aya Mahfouz <mahfouz.saif.elyazal@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>