mtd: spi-nor: Fix divide by zero for spi-nor-generic flashes

We failed to initialize n_banks for spi-nor-generic flashes, which
caused a devide by zero when computing the bank_size.

By default we consider that all chips have a single bank. Initialize
the default number of banks for spi-nor-generic flashes. Even if the
bug is fixed with this simple initialization, check the n_banks value
before dividing so that we make sure this kind of bug won't occur again
if some other struct instance is created uninitialized.

Suggested-by: Todd Brandt <todd.e.brandt@linux.intel.com>
Reported-by: Todd Brandt <todd.e.brandt@linux.intel.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217448
Fixes: 9d6c5d64

 ("mtd: spi-nor: Introduce the concept of bank")
Link: https://lore.kernel.org/all/20230516225108.29194-1-todd.e.brandt@intel.com/
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Tested-by: Todd Brandt <todd.e.brandt@linux.intel.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230518085440.2363676-1-tudor.ambarus@linaro.org