tty: tty_ldisc: Supply missing description for 'tty_ldisc_get's 'tty' param

tty: tty_ldisc: Supply missing description for 'tty_ldisc_get's 'tty' param

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

 drivers/tty/tty_ldisc.c:160: warning: Function parameter or member 'tty' not described in 'tty_ldisc_get'

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/20201104193549.4026187-30-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>