tty: n_gsm: remove unneeded initialization of ret in gsm_dlci_config

tty: n_gsm: remove unneeded initialization of ret in gsm_dlci_config

The variable 'ret' is not used before assignment from gsm_activate_mux().
Still it gets initialized to zero at declaration.

Fix this as remarked in the link below by moving the declaration to the
first assignment.

Link: https://lore.kernel.org/all/b42bc4d1-cc9d-d115-c981-aaa053bdc59f@kernel.org/
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230817093231.2317-3-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>