iio: accel: mma7660: fix non static symbol warning

Fixes the following sparse warning:

drivers/iio/accel/mma7660.c:42:11: warning:
 symbol 'mma7660_nscale' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>