iio: accel: BMC150: fix issues when CONFIG_PM_RUNTIME is not set

When CONFIG_PM_RUNTIME is not set, the following issues are seen:
 * warning message at compilation time:
    warning: 'bmc150_accel_get_startup_times' defined but not used [-Wunused-function]
 * bmc150_accel_set_power_state() will always fail and reading the
   accelerometer data is impossible;
   This occurs because of the call to pm_runtime_put_autosuspend calls
   __pm_runtime_suspend which returns -ENOSYS.

This commit fixes these.

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>