Some lpm implementation do not follow docs

[lebrush]

The LPM notification indicates that the lpm_arch_set method shall return the mode which was actually set. However, some implementations (stm32f2, stm32f1, any maybe others) return the previous mode to the one which was set.

 64 /**
 65  * @brief Try to set the controller to a given power mode
 66  *
 67  * @param[in] target    the desired power mode
 68  *
 69  * @return              the power mode that was actually set
 70  */
 71 enum lpm_mode lpm_arch_set(enum lpm_mode target);

Either the documentation needs to be updated or the code fixed.
This probably expires once #6160 (or relevant) is merged.

[lebrush]

This can be closed since #6160 got merged.