tty: switch tty_{,un}throttle_safe() to return a bool

They return 0 or 1 -- a boolean value, so make it clear than noone
should expect negative or other values.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230919085156.1578-7-jirislaby@kernel.org


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