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

Fix below division by zero warning:
- Added an if statement because buswidth can be zero, resulting in division by zero.
- The modified code was based on another driver (atmel-quadspi).

[    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: 7512eaf5

 ("spi: cadence-quadspi: Fix dummy cycle calculation when buswidth > 1")
Signed-off-by: Yoshitaka Ikeda <ikeda@nskint.co.jp>
Link: https://lore.kernel.org/r/ed989af6-da88-4e0b-9ed8-126db6cad2e4@nskint.co.jp
Signed-off-by: Mark Brown <broonie@kernel.org>