tty: vt: make types of screenpos() more consistent

* parameter offset: it is expected to be non-negative, so switch to
  unsigned
* return type: switch from ushort to explicit u16. This is expected on
  most places. And fix the remaining two places too.

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/20240122110401.7289-42-jirislaby@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>