tty: n_gsm: remove redundant pointer gsm

Pointer gsm is assigned a value that is never read, hence it is
redundant and can be removed. Cleans up clang warning:

drivers/tty/n_gsm.c:2979:2: warning: Value stored to 'gsm' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>