iio:accel:bma400: Move exports into IIO_BMA400 namespace

In order to avoid unnecessary pollution of the global symbol namespace
move the core bma400 functions into a bma400 specific namespace
and import that into the two bus modules.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220116180535.2367780-5-jic23@kernel.org