iio:accel:bmc150: Mark structure __maybe_unused as only needed with for pm ops.

If CONFIG_PM not set then clang warns this structure is unused.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20211128172445.2616166-12-jic23@kernel.org