spi: dw: De-assert chip-select on reset

SPI memory operations implementation will require to have the CS register
cleared before executing the operation in order not to have the
transmission automatically started prior the Tx FIFO is pre-initialized.
Let's clear the register then on explicit controller reset to fulfil the
requirements in case of an error or having the CS left set by a bootloader
or another software.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20201007235511.4935-14-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-dw.h

@@ -238,15 +238,16 @@ static inline void spi_umask_intr(struct dw_spi *dws, u32 mask)
 }
 
 /*
- * This disables the SPI controller, interrupts, clears the interrupts status,
- * and re-enable the controller back. Transmit and receive FIFO buffers are
- * cleared when the device is disabled.
+ * This disables the SPI controller, interrupts, clears the interrupts status
+ * and CS, then re-enables the controller back. Transmit and receive FIFO
+ * buffers are cleared when the device is disabled.
  */
 static inline void spi_reset_chip(struct dw_spi *dws)
 {
 	spi_enable_chip(dws, 0);
 	spi_mask_intr(dws, 0xff);
 	dw_readl(dws, DW_SPI_ICR);
+	dw_writel(dws, DW_SPI_SER, 0);
 	spi_enable_chip(dws, 1);
 }