tty: tty_ldisc: fix doc warnings in tty_ldisc.c

tty: tty_ldisc: fix doc warnings in tty_ldisc.c

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

 drivers/tty/tty_ldisc.c:63: warning: Excess function
  parameter 'disc' description in 'tty_register_ldisc'
 drivers/tty/tty_ldisc.c:90: warning: Function parameter or
  member 'ldisc' not described in 'tty_unregister_ldisc'
 drivers/tty/tty_ldisc.c:90: warning: Excess function
  parameter 'disc' description in 'tty_unregister_ldisc'

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Link: https://lore.kernel.org/r/20210608024843.2756746-1-libaokun1@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>