mtd: spi-nor: spansion: let SFDP determine the flash and sector size

sector_size is used to determine the flash size and the erase size in
case of uniform erase. n_sectors is used to determine the flash_size.
But the flash size and the erase sizes are determined when parsing SFDP,
let SFDP determine them.

Tested-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Link: https://lore.kernel.org/r/20230726075257.12985-10-tudor.ambarus@linaro.org
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>