mtd: spi-nor: convert .n_sectors to .size

mtd: spi-nor: convert .n_sectors to .size

.n_sectors is rarely used. In fact it is only used in swp.c and to
calculate the flash size in the core. The use in swp.c might be
converted to use the (largest) flash erase size. For now, we just
locally calculate the sector size.

Simplify the flash_info database and set the size of the flash directly.
This also let us use the SZ_x macros.

Verified that there's no flash that specifies BP and sector size of zero
to make sure we avoid a division by zero in
spi_nor_get_min_prot_length_sr(). We'll protect from a possible division
by zero in a further patch by introducing a default value for
sector_size.

Signed-off-by: Michael Walle <mwalle@kernel.org>
Link: https://lore.kernel.org/r/20230807-mtd-flash-info-db-rework-v3-5-e60548861b10@kernel.org
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>