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 <[e-mail]>
Cc: Jiri Slaby <[e-mail]>
Cc: [e-mail]
Signed-off-by: Lee Jones <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/synclink_gt.c

···
 	return 0;
 }
 
-/**
+/*
  * line discipline callback wrappers
  *
  * The wrappers maintain line discipline references
···
 /**
  * hdlcdev_tx_timeout - called by network layer when transmit timeout is detected
  * @dev: pointer to network device structure
+ * @txqueue: unused
  */
 static void hdlcdev_tx_timeout(struct net_device *dev, unsigned int txqueue)
 {