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

In the Linux kernel, the following vulnerability has been resolved:

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