iio: imu: bmi270: move private struct declaration to source file

iio: imu: bmi270: move private struct declaration to source file

The device's private data struct is currently declared in the header
file, but it does not need to be exposed there. Move it to the driver's
core source file to avoid unnecessary #include directives or forward
declarations in the header.

Signed-off-by: Gustavo Silva <gustavograzs@gmail.com>
Acked-by: Alex Lanzano <lanzano.alex@gmail.com>
Link: https://patch.msgid.link/20250219-bmi270-irq-v1-1-145d02bbca3b@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>