spi: use specific last_cs instead of last_cs_enable

Commit d40f0b6f2e instroduced last_cs_enable to avoid setting
chipselect if it's not necessary, but it also introduces a bug. The
chipselect may not be set correctly on multi-device SPI busses. The
reason is that we can't judge the chipselect by bool last_cs_enable,
since chipselect may be modified after other devices were accessed.

So we should record the specific state of chipselect in case of
confusion.

Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
Link: https://lore.kernel.org/r/20220217141234.72737-1-yun.zhou@windriver.com
Signed-off-by: Mark Brown <broonie@kernel.org>

include/linux/spi/spi.h

@@ -370,7 +370,8 @@ extern struct spi_device *spi_new_ancillary_device(struct spi_device *spi, u8 ch
  * @cur_msg_prepared: spi_prepare_message was called for the currently
  *                    in-flight message
  * @cur_msg_mapped: message has been mapped for DMA
- * @last_cs_enable: was enable true on the last call to set_cs.
+ * @last_cs: the last chip_select that is recorded by set_cs, -1 on non chip
+ *           selected
  * @last_cs_mode_high: was (mode & SPI_CS_HIGH) true on the last call to set_cs.
  * @xfer_completion: used by core transfer_one_message()
  * @busy: message pump is busy
@@ -603,7 +604,7 @@ struct spi_controller {
 	bool				auto_runtime_pm;
 	bool                            cur_msg_prepared;
 	bool				cur_msg_mapped;
-	bool				last_cs_enable;
+	char				last_cs;
 	bool				last_cs_mode_high;
 	bool                            fallback;
 	struct completion               xfer_completion;