tty: tty_buffer: Add missing description for 'limit'

Fixes the following W=1 kernel build warning(s):

 drivers/tty/tty_buffer.c:592: warning: Function parameter or member 'limit' not described in 'tty_buffer_set_limit'

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104193549.4026187-6-lee.jones@linaro.org


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