If the DAC peripheral has a mode register (DAC_MCR), it is set to normal mode with buffer enabled and connected to external pin and on-chip peripherals. This allows to measure the current value of a DAC channel or to use the DAC channel also for other on-chip peripherals.
For boards that have not connected the V_REF+ pin to an external reference voltage, the VREFBUF peripheral can be used as V_REF+ if supported by setting `VREFBUF_ENABLE=1`.
The current implmentation right shifted the 16 bit value passed into
`dac_set()` down to the 12 bits that the DAC is actually capable of.
This patch drops the shift and instead writes the 16 bit value to the
DAC's left aligned 12 bit wide data holding register.
