iio: magnetometer: mag3110: Add ability to run in continuous mode

Adds the ability to run the Mag3110 in continuous mode to speed up the
sampling rate.
Depending on the sampling rate requested the device can be put in or out
of continuous mode automatically.
Shifting out of continuous mode requires a potential 1 / ODR wait which
is also implemented.
Modified the sleep method when data is not ready to allow for
sampling > 50sps to work.

Signed-off-by: Richard Tresidder <rtresidd@electromag.com.au>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>