mtd: spi-nor: Avoid forward declaration of internal functions

Reorganize the code to kill forward declarations of spi_nor_match_id()
macronix_quad_enable() and spi_nor_hwcaps_read2cmd().
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>