This commit adds two new functions to the DMA peripheral code for the
stm32. The setup function allows for a one-time setup of peripheral
config. The prepare function does the per-transfer setup. This allows
for a single setup call during the peripheral lock step and a
per-transfer call to the prepare function.
This commit removes a number of assert statements that should already
have been hit before. This is the reason that the assert in the
acquire function is left.
The FCR register content might change during mem-to-mem DMA transfers,
Forcing it back in the acquire should be sufficient to ensure proper
operations.
