iio: magn: bmc150_magn: do not set power state twice when setting trigger state

When setting the trigger state, the device power state is set through
buffer preenable and postdisable hooks. There is no need
to also set it in the trigger set state call.

Remove duplicate set power state from the trigger set
state call.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>