mtd: spi-nor: core: Fix erase type discovery for overlaid region

mtd: spi-nor: core: Fix erase type discovery for overlaid region

In case of overlaid regions in which their biggest erase size command
overpasses in size the region's size, only the non-overlaid portion of
the sector gets erased. For example, if a Sector Erase command is applied
to a 256-kB range that is overlaid by 4-kB sectors, the overlaid 4-kB
sectors are not affected by the erase.
For overlaid regions, 'region->size' is assigned to 'cmd->size' later in
spi_nor_init_erase_cmd(), so 'erase->size' can be greater than 'len'.

Fixes: 5390a8df769e ("mtd: spi-nor: add support to non-uniform SFDP SPI NOR flash memories")
Cc: stable@vger.kernel.org
Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
[ta: Update commit description, add Fixes tag and Cc to stable]
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/fa5d8b944a5cca488ac54ba37c95e775ac2deb34.1601612872.git.Takahiro.Kuwano@infineon.com