mtd: spi-nor: sst: fix BPn bits for the SST25VF064C

This flash part actually has 4 block protection bits.

Please note, that this patch is just based on information of the
datasheet of the datasheet and wasn't tested.

Fixes: 3e0930f1 ("mtd: spi-nor: Rework the disabling of block write protection")
Reported-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20201203162959.29589-2-michael@walle.cc