tty: synclinkmp: Add missing description for function param 'txqueue'

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

 drivers/tty/synclinkmp.c:1808: 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/20201104193549.4026187-27-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/synclinkmp.c

@@ -1803,6 +1803,7 @@ static int hdlcdev_ioctl(struct net_device *dev, struct ifreq *ifr, int cmd)
 /**
  * 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)
 {