tty: synclink_gt: Demote one kernel-doc header and repair another

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

 drivers/tty/synclink_gt.c:633: warning: Function parameter or member 'tty' not described in 'ldisc_receive_buf'
 drivers/tty/synclink_gt.c:633: warning: Function parameter or member 'data' not described in 'ldisc_receive_buf'
 drivers/tty/synclink_gt.c:633: warning: Function parameter or member 'flags' not described in 'ldisc_receive_buf'
 drivers/tty/synclink_gt.c:633: warning: Function parameter or member 'count' not described in 'ldisc_receive_buf'
 drivers/tty/synclink_gt.c:1683: warning: Function parameter or member 'txqueue' not described in 'hdlcdev_tx_timeout'

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: paulkf@microgate.com
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104193549.4026187-24-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>