iio: accel: bma180: Fix BMA25x bandwidth register values

According to the BMA253 datasheet [1] and BMA250 datasheet [2] the
bandwidth value for BMA25x should be set as 01xxx:

  "Settings 00xxx result in a bandwidth of 7.81 Hz; [...]
   It is recommended [...] to use the range from ´01000b´ to ´01111b´
   only in order to be compatible with future products."

However, at the moment the drivers sets bandwidth values from 0 to 6,
which is not recommended and always results into 7.81 Hz bandwidth
according to the datasheet.

Fix this by introducing a bw_offset = 8 = 01000b for BMA25x,
so the additional bit is always set for BMA25x.

[1]: https://www.bosch-sensortec.com/media/boschsensortec/downloads/datasheets/bst-bma253-ds000.pdf
[2]: https://datasheet.octopart.com/BMA250-Bosch-datasheet-15540103.pdf

Cc: Peter Meerwald <pmeerw@pmeerw.net>
Fixes: 2017cff2

 ("iio:bma180: Add BMA250 chip support")
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210526094408.34298-2-stephan@gerhold.net
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>