spi: Add an helper to flush the message queue

This is needed by the spi-mem logic to force all messages that have been
queued before a memory operation to be sent before we start the memory
operation. We do that in order to guarantee that spi-mem operations do
not preempt regular SPI transfers.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

spi: Expose spi_{map,unmap}_buf() for internal use

spi_{map,unmap}_buf() are needed by the spi-mem logic that is about to
be introduced to prepare data buffer for DMA operations.

Remove the static specifier on these functions and add their prototypes
to drivers/spi/internals.h. We do not export the symbols here because
both SPI_MEM and SPI can't be enabled as modules and we'd like to
prevent controller/device drivers from using these functions.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>