iio: mma8452: Initialise before activating

Many of the hardware configuration registers may only be modified while the
device is inactive.

Currently the probe code first activates the device and then modifies the
registers (eg to set the scale). This doesn't actually work but is not
noticed since the scale used is the default value.

While at it also issue a hardware reset command at probe time.

Signed-off-by: Martin Fuzzey <mfuzzey@parkeon.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>