spi: zynqmp_gqspi: Fix write issue at low frequencies

With current implementation we are seeing write issues at low frequencies
below 15Mhz. Make below changes to fix the issue.

1. Remove dummy genfifo entry in zynqmp_qspi_chipselect() which was
   incorrectly added in the past

2. Enable and poll for TX_FIFO_Empty after Tx data is filled in FIFO in
   zynqmp_qspi_fill_tx_fifo().

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Link: https://lore.kernel.org/r/1634652780-21755-1-git-send-email-ashok.reddy.soma@xilinx.com
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

drivers/spi/zynqmp_gqspi.c

@@ -37,6 +37,7 @@
  */
 #define GQSPI_IXR_TXNFULL_MASK		0x00000004 /* QSPI TX FIFO Overflow */
 #define GQSPI_IXR_TXFULL_MASK		0x00000008 /* QSPI TX FIFO is full */
+#define GQSPI_IXR_TXFIFOEMPTY_MASK	0x00000100 /* QSPI TX FIFO is Empty */
 #define GQSPI_IXR_RXNEMTY_MASK		0x00000010 /* QSPI RX FIFO Not Empty */
 #define GQSPI_IXR_GFEMTY_MASK		0x00000080 /* QSPI Generic FIFO Empty */
 #define GQSPI_IXR_GFNFULL_MASK		0x00000200 /* QSPI GENFIFO not full */
@@ -279,9 +280,6 @@ static void zynqmp_qspi_chipselect(struct zynqmp_qspi_priv *priv, int is_on)
 
 	debug("GFIFO_CMD_CS: 0x%x\n", gqspi_fifo_reg);
 
-	/* Dummy generic FIFO entry */
-	zynqmp_qspi_fill_gen_fifo(priv, 0);
-
 	zynqmp_qspi_fill_gen_fifo(priv, gqspi_fifo_reg);
 }
 
@@ -470,6 +468,13 @@ static int zynqmp_qspi_fill_tx_fifo(struct zynqmp_qspi_priv *priv, u32 size)
 		}
 	}
 
+	ret = wait_for_bit_le32(&regs->isr, GQSPI_IXR_TXFIFOEMPTY_MASK, 1,
+				GQSPI_TIMEOUT, 1);
+	if (ret) {
+		printf("%s: Timeout\n", __func__);
+		return ret;
+	}
+
 	priv->tx_buf += len;
 	return 0;
 }