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>

drivers/iio/accel/mma7660.c

@@ -39,7 +39,7 @@
 
 #define MMA7660_SCALE_AVAIL	"0.467142857"
 
-const int mma7660_nscale = 467142857;
+static const int mma7660_nscale = 467142857;
 
 #define MMA7660_CHANNEL(reg, axis) {	\
 	.type = IIO_ACCEL,	\