tty: n_tty: Add 2 missing parameter descriptions

tty: n_tty: Add 2 missing parameter descriptions

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

 drivers/tty/n_tty.c:405: warning: Function parameter or member 'tty' not described in 'is_continuation'
 drivers/tty/n_tty.c:1701: warning: Function parameter or member 'flow' not described in 'n_tty_receive_buf_common'

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: "Andrew J. Kroll" <ag784@freenet.buffalo.edu>
Cc: processes-Sapan Bhatia <sapan@corewars.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104193549.4026187-11-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>