mtd: sf_spi: Use proper return types

Signed-off-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
mtd: sf_ops: Adjust sst write with sf_nor

Signed-off-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>