tty: n_hdlc: Fix a little doc-rot in n_hdlc_tty_read()

tty: n_hdlc: Fix a little doc-rot in n_hdlc_tty_read()

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

 drivers/tty/n_hdlc.c:421: warning: Function parameter or member 'kbuf' not described in 'n_hdlc_tty_read'
 drivers/tty/n_hdlc.c:421: warning: Function parameter or member 'cookie' not described in 'n_hdlc_tty_read'
 drivers/tty/n_hdlc.c:421: warning: Function parameter or member 'offset' not described in 'n_hdlc_tty_read'
 drivers/tty/n_hdlc.c:421: warning: Excess function parameter 'buf' description in 'n_hdlc_tty_read'

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Paul Fulghum <paulkf@microgate.com>
Cc: Michael Callahan <callahan@maths.ox.ac.uk>
Cc: Al Longyear <longyear@netcom.com>
Cc: Paul Mackerras <Paul.Mackerras@cs.anu.edu.au>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210520121906.3468725-9-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>