spi: cadence_qspi: Fix baud rate calculation

With the existing code, when the requested SPI clock rate is near
to the lowest that can be achieved by the hardware (max divider
of the ref clock is 32), the generated clock rate is wrong.
For example, with a 50MHz ref clock, when asked for anything less
than a 1.5MHz SPI clock, the code sets up the divider to generate
25MHz.

This change fixes the calculation.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>

drivers/spi/cadence_qspi_apb.c

@@ -273,22 +273,12 @@ void cadence_qspi_apb_config_baudrate_div(void *reg_base,
 	reg = readl(reg_base + CQSPI_REG_CONFIG);
 	reg &= ~(CQSPI_REG_CONFIG_BAUD_MASK << CQSPI_REG_CONFIG_BAUD_LSB);
 
-	div = ref_clk_hz / sclk_hz;
-
-	if (div > 32)
-		div = 32;
-
-	/* Check if even number. */
-	if ((div & 1)) {
-		div = (div / 2);
-	} else {
-		if (ref_clk_hz % sclk_hz)
-			/* ensure generated SCLK doesn't exceed user
-			specified sclk_hz */
-			div = (div / 2);
-		else
-			div = (div / 2) - 1;
-	}
+	/*
+	 * The baud_div field in the config reg is 4 bits, and the ref clock is
+	 * divided by 2 * (baud_div + 1). Round up the divider to ensure the
+	 * SPI clock rate is less than or equal to the requested clock rate.
+	 */
+	div = DIV_ROUND_UP(ref_clk_hz, sclk_hz * 2) - 1;
 
 	debug("%s: ref_clk %dHz sclk %dHz Div 0x%x\n", __func__,
 	      ref_clk_hz, sclk_hz, div);