spi: ti_qspi: Add dummy readl for bus sync

Add dummy readl after invalidating cmd field of QSPI_CMD_REG to ensure
bus sync. Without this device's CS is not deactivated reliably leading
to failure to enumerate flash or failure to set quad enable bit on
Macronix flash present on am437x-sk and am437x-idk evms.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>

drivers/spi/ti_qspi.c

@@ -170,6 +170,8 @@ void spi_cs_deactivate(struct spi_slave *slave)
 	debug("spi_cs_deactivate: 0x%08x\n", (u32)slave);
 
 	writel(qslave->cmd | QSPI_INVAL, &qslave->base->cmd);
+	/* dummy readl to ensure bus sync */
+	readl(&qslave->base->cmd);
 }
 
 void spi_init(void)