n_gsm: Use array_index_nospec() with index that comes from userspace

n_gsm: Use array_index_nospec() with index that comes from userspace

dc.channel used for indexing comes directly from copy_from_user(). Use
array_index_nospec() to mitigate speculative side-channel.

Link: https://lore.kernel.org/linux-serial/64306d13.ONswMlyWlVKLGkKR%25lkp@intel.com/
Cc: stable <stable@kernel.org>
Fixes: afe3154ba87e ("tty: n_gsm: add ioctl for DLC config via ldisc handle")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Daniel Starke <daniel.starke@siemens.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230411164532.64175-1-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>