tty: n_gsm: Add some instructions and code for requester

The gsm driver can configure initiator or requester by parameter
initiator,but the config code and using are different ,the doc has
initiator instructions only,it should be add instructions for requester.

Signed-off-by: Zhenguo Zhao <Zhenguo.Zhao1@unisoc.com>
Link: https://lore.kernel.org/r/1629461872-26965-1-git-send-email-zhenguo6858@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>