tty: hvcs: make ktermios const

Make this const as it is not modified anywhere.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Reviewed-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>