The device ID is defined as a uint32_t in the driver and topic but
stored as a int32_t param. It is therefore sufficient if the device ID
is not 0 in order to be valid.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

Add required rotation option to driver startup command and fix a couple of details in warnings/comments

change type mag_scale to type struct mag_calibration_s for compatibility with master (only noted this thanks to travis build test)