tty: make tty_buffer_space_avail return uint

tty_buffer_space_avail returns values >= 0, so make it clear by the
return type.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20210505091928.22010-25-jslaby@suse.cz


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