tty: n_gsm: fix redundant assignment of gsm->encoding

The function gsmld_open() contains a redundant assignment of gsm->encoding.
The same value of GSM_ADV_OPT is already assigned to it during the
initialization of the struct in gsm_alloc_mux() a few lines earlier.

Fix this by removing the redundant second assignment of gsm->encoding in
gsmld_open().

Fixes: e1eaea46

 ("tty: n_gsm line discipline")
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230420085017.7314-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>