mtd: spi-nor: Pointer parameter for SR in spi_nor_read_sr()

Let the callers pass the pointer to the DMA-able buffer where
the value of the Status Register will be written. This way we
avoid the casts between int and u8, which can be confusing.

Callers stop compare the return value of spi_nor_read_sr() with negative,
spi_nor_read_sr() returns 0 on success and -errno otherwise.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>