mtd: spi-nor: Pointer parameter for FSR in spi_nor_read_fsr()

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

Caller stops compare the return value of spi_nor_read_fsr() with negative,
spi_nor_read_fsr() returns 0 on success and -errno otherwise.

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