Let the callers pass the pointer to the DMA-able buffer where
the value of the Configuration 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_cr() with negative,
spi_nor_read_cr() returns 0 on success and -errno otherwise.
Signed-off-by: NTudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: NBoris Brezillon <boris.brezillon@collabora.com>