mtd: spi-nor-core: allow truncated erases

mtd: spi-nor-core: allow truncated erases

On devices with non-uniform sector sizes like Spansion S25 or S28 family
of flashes the sector under erase does not necessarily have to be
mtd->erasesize bytes long. For example, on S28 flashes the first 128 KiB
region is composed of 32 4 KiB sectors, then a 128 KiB sector, and then
256 KiB sectors till the end.

Let the flash-specific erase functions erase less than the requested
length in case of the 4 or 128 KiB sectors and report the number of
bytes erased back to the calling function.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>