We cannot just decrement the reference counter of power modes without
any coordination. First, this will trigger an `assert()`ion on non
STM32 MCUs that have power modes that are not used (the ref count would
be decremented below zero). Second, there hopefully is a reason a
certain power mode is blocked, e.g. because a periph driver needs a
certain clock to function.
Likely the `periph_uart` driver on STM32 boards keeps power modes
blocked after TX is completed even when no RX callback is present, which
is the waste of power this hack tries to address. But that should be
addressed there.
This is already unblocked and will trigger an assertion.
The code is still broken as other modes might be unblocked too,
but at least it is just as broken as it was before #17895
