iio: imu: inv_mpu6050: make arrays hz and d static

Don't populate the arrays on the stack, instead make them static.
Makes the object code smaller by 135 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  15135	   4240	    128	  19503	   4c2f	inv_mpu_core.o

After:
   text	   data	    bss	    dec	    hex	filename
  14840	   4400	    128	  19368	   4ba8	inv_mpu_core.o

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>