main: Fix scale handling in IIO drivers

Our code expects 256 to corresponds to 1G acceleration.
IIO drivers give out a value that, when multiplied by the scale, gives
an acceleration in m/s².

Do the math to transform the IIO sensor's value to the scale we use
internally.
main: Fix unchanging accel values in polling driver

g_udev_device_get_sysfs_attr_as_int() uses libudev internally, which
does caching. This means that the values wouldn't have changed past
the first read.