tty: localise ptychar and make it const

ptychar was not const, so mark it as such. And move this variable to the
only place where it's used.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210302062214.29627-36-jslaby@suse.cz


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