mtd: spi-nor: Void return type for spi_nor_clear_sr/fsr()

spi_nor_clear_sr() and spi_nor_clear_fsr() are called just in case
of errors. The callers didn't check their return value, make them
of type void.

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