spi: spi-cadence-quadspi: Fix division by zero warning

Fix below division by zero warning:
- The reason for dividing by zero is because the dummy bus width is zero,
  but if the dummy n bytes is zero, it indicates that there is no data transfer,
  so we can just return zero without doing any calculations.

[    0.795337] Division by zero in kernel.
   :
[    0.834051] [<807fd40c>] (__div0) from [<804e1acc>] (Ldiv0+0x8/0x10)
[    0.839097] [<805f0710>] (cqspi_exec_mem_op) from [<805edb4c>] (spi_mem_exec_op+0x3b0/0x3f8)

Fixes: 7512eaf541 ("spi: cadence-quadspi: Fix dummy cycle calculation when buswidth > 1")
Signed-off-by: Yoshitaka Ikeda <ikeda@nskint.co.jp>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/92eea403-9b21-2488-9cc1-664bee760c5e@nskint.co.jp
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-cadence-quadspi.c

@@ -309,6 +309,9 @@ static unsigned int cqspi_calc_dummy(const struct spi_mem_op *op, bool dtr)
 {
 	unsigned int dummy_clk;
 
+	if (!op->dummy.nbytes)
+		return 0;
+
 	dummy_clk = op->dummy.nbytes * (8 / op->dummy.buswidth);
 	if (dtr)
 		dummy_clk /= 2;