mtd: spi-nor: Check for zero erase size in spi_nor_find_best_erase_type()

mtd: spi-nor: Check for zero erase size in spi_nor_find_best_erase_type()

Erase can be zeroed in spi_nor_parse_4bait() or
spi_nor_init_non_uniform_erase_map(). In practice it happened with
mt25qu256a, which supports 4K, 32K, 64K erases with 3b address commands,
but only 4K and 64K erase with 4b address commands.

Fixes: dc92843159a7 ("mtd: spi-nor: fix erase_type array to indicate current map conf")
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20211119081412.29732-1-alexander.sverdlin@nokia.com