tty: vt: add con_putc() helper

And let it call consw::con_putc() if it exists, otherwise
consw::con_putcs(). This is similar to tty_put_char().

It supports dropping unneeded duplication of code like sticon_putc() is
(see the next patch).

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/20240122110401.7289-24-jirislaby@kernel.org


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