iio: Correct description how to convert microtesla to gauss

this just fixes the comment; however, I'm not sure if the driver reports
measurements correctly; the raw values are 0.3 uT / LSB; IIO is supposed
to report magnetic fields in Gauss, so the scale should be around 1/300
(ignoring ASA) -- but value and scale are returned as VAL_INT

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>