mtd: spi-nor: Merge spi_nor_write_sr() and spi_nor_write_sr_cr()

Merge
static int spi_nor_write_sr(struct spi_nor *nor, u8 val)
static int spi_nor_write_sr_cr(struct spi_nor *nor, const u8 *sr_cr)
into
static int spi_nor_write_sr(struct spi_nor *nor, const u8 *sr, size_t len)

The Status Register can be written with one or two bytes. Merge
the two functions to avoid code duplication.

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