Previously, the /CS signal was performed by enabling / disabling the
SPI peripheral. This had the disadvantage that clock polarity settings
where not applied starting with `spi_acquire()`, as assumed by e.g.
the SPI SD card driver, but only just before transmitting data.
Now the SPI peripheral is enabled on `spi_acquire()` and only disabled
when calling `spi_release()`, and the `SPI_CR2_SSOE` bit in the `CR2`
register is used for hardware /CS handling (as supposed to).
The CR2 register was only written to if the settings differ from the
reset value. This wasn't actually a bug, since it was cleared in
`spi_release()` to the reset value again. Still, it looks like a bug,
may cause a pipeline flush due to the branch, and increased `.text`
size. So let's get rid of this.
The driver previously failed to reliably clear the RXNE bit, resulting
in the next transfer to incorrectly read a stale register value. This
was noticed with the SD card SPI driver on an STM32F4, in which the
0xff byte of the previous byte transfer was returned instead of the
actual status byte, throwing the SD card driver off the rails.
The DMA stream will automatically disable itself as soon as the transfer
is finished. No need to do this an additional time after the transfer is
finished
This combines a number of register writes in the SPI
acquire and transfer code. The DMA enable for SPI is moved to the
acquire function, switching between DMA and regular transfer between
acquires is not possible.
