tty: n_gsm: Fix function naming and provide missing param descriptions

tty: n_gsm: Fix function naming and provide missing param descriptions

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

 drivers/tty/n_gsm.c:525: warning: expecting prototype for gsm_stuff_packet(). Prototype was for gsm_stuff_frame() instead
 drivers/tty/n_gsm.c:1608: warning: expecting prototype for gsm_dlci_control(). Prototype was for gsm_dlci_command() instead
 drivers/tty/n_gsm.c:2561: warning: Function parameter or member 'cookie' not described in 'gsmld_read'
 drivers/tty/n_gsm.c:2561: warning: Function parameter or member 'offset' not described in 'gsmld_read'

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/20210520121906.3468725-11-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>