tty: n_gsm: cleanup gsm_control_command and gsm_control_reply

tty: n_gsm: cleanup gsm_control_command and gsm_control_reply

There are multiple places in gsm_control_command and gsm_control_reply that
derive the specific DLCI handle directly out of the DLCI table in gsm.

Add a local variable which holds this handle and use it instead to improve
code readability.

Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230817093231.2317-7-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>