iio:magnetometer:bmc150_magn: output intended variable

According to the debug/error string, the content of chip_id is supposed to
be output, rather than the return value of the previous operation.

Fixes: c91746a2 ("iio: magn: Add support for BMC150 magnetometer")

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>