iio: accel: bma180: Rename center_temp to temp_offset

The bma180 driver is being extended to support the bma150.
Its temperature channel is unsigned so the center_temp naming
no longer makes.

Signed-off-by: Jonathan Bakker <xc-racer2@live.ca>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>