tty: n_gsm: Demote obvious abuse of kernel-doc and supply other missing docss

tty: n_gsm: Demote obvious abuse of kernel-doc and supply other missing docss

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

 drivers/tty/n_gsm.c:85: warning: Function parameter or member 'ref' not described in 'gsm_mux_net'
 drivers/tty/n_gsm.c:85: warning: Function parameter or member 'dlci' not described in 'gsm_mux_net'
 drivers/tty/n_gsm.c:664: warning: Function parameter or member 'dlci' not described in 'gsm_data_kick'
 drivers/tty/n_gsm.c:1015: warning: Function parameter or member 'clen' not described in 'gsm_process_modem'

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104193549.4026187-19-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>