tty: vt: sanitize consw::con_putcs() parameters

tty: vt: sanitize consw::con_putcs() parameters

Similar to con_putc() in the previous patch:
* make the pointer to charattr a pointer to u16, and
* make x, y, and count unsigned as they are strictly non-negative.

And again, document that hook.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Tested-by: Helge Deller <deller@gmx.de> # parisc STI console
Link: https://lore.kernel.org/r/20240122110401.7289-27-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>