iio: gyro: bmg160: only set power state if PM_RUNTIME is defined

When CONFIG_PM_RUNTIME is not defined and bmg160 tries to power
off the device, bmg160_set_power_state will call pm_runtime_put_autosuspend,
which is not implemented (wil return -ENOSYS).

Only call bmg160_set_power_state when CONFIG_PM_RUNTIME is defined.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>