spi: spi-mem: Add fix to avoid divide error

For some SPI flash memory operations, dummy bytes are not mandatory. For
example, in Winbond SPINAND flash memory devices, the `write_cache` and
`update_cache` operation variants have zero dummy bytes. Calculating the
duration for SPI memory operations with zero dummy bytes causes
a divide error when `ncycles` is calculated in the
spi_mem_calc_op_duration().

Add changes to skip the 'ncylcles' calculation for zero dummy bytes.

Following divide error is fixed by this change:

 Oops: divide error: 0000 [#1] PREEMPT SMP NOPTI
...

  ? do_trap+0xdb/0x100
  ? do_error_trap+0x75/0xb0
  ? spi_mem_calc_op_duration+0x56/0xb0
  ? exc_divide_error+0x3b/0x70
  ? spi_mem_calc_op_duration+0x56/0xb0
  ? asm_exc_divide_error+0x1b/0x20
  ? spi_mem_calc_op_duration+0x56/0xb0
  ? spinand_select_op_variant+0xee/0x190 [spinand]
  spinand_match_and_init+0x13e/0x1a0 [spinand]
  spinand_manufacturer_match+0x6e/0xa0 [spinand]
  spinand_probe+0x357/0x7f0 [spinand]
  ? kernfs_activate+0x87/0xd0
  spi_mem_probe+0x7a/0xb0
  spi_probe+0x7d/0x130

Fixes: 226d6cb ("spi: spi-mem: Estimate the time taken by operations")
Suggested-by: Krishnamoorthi M <krishnamoorthi.m@amd.com>
Co-developed-by: Akshata MukundShetty <akshata.mukundshetty@amd.com>
Signed-off-by: Akshata MukundShetty <akshata.mukundshetty@amd.com>
Signed-off-by: Raju Rangoju <Raju.Rangoju@amd.com>
Link: https://patch.msgid.link/20250424121333.417372-1-Raju.Rangoju@amd.com
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-mem.c

@@ -596,7 +596,11 @@ u64 spi_mem_calc_op_duration(struct spi_mem_op *op)
 	ns_per_cycles = 1000000000 / op->max_freq;
 	ncycles += ((op->cmd.nbytes * 8) / op->cmd.buswidth) / (op->cmd.dtr ? 2 : 1);
 	ncycles += ((op->addr.nbytes * 8) / op->addr.buswidth) / (op->addr.dtr ? 2 : 1);
-	ncycles += ((op->dummy.nbytes * 8) / op->dummy.buswidth) / (op->dummy.dtr ? 2 : 1);
+
+	/* Dummy bytes are optional for some SPI flash memory operations */
+	if (op->dummy.nbytes)
+		ncycles += ((op->dummy.nbytes * 8) / op->dummy.buswidth) / (op->dummy.dtr ? 2 : 1);
+
 	ncycles += ((op->data.nbytes * 8) / op->data.buswidth) / (op->data.dtr ? 2 : 1);
 
 	return ncycles * ns_per_cycles;