mtd: spi-nor: mark desirable switch case fall through

gcc 7 with -Wimplicit-fallthrough raises:

drivers/mtd/spi-nor/spi-nor.c: In function ‘set_4byte’:
drivers/mtd/spi-nor/spi-nor.c:289:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
   need_wren = true;
   ~~~~~~~~~~^~~~~~
drivers/mtd/spi-nor/spi-nor.c:290:2: note: here
  case SNOR_MFR_MACRONIX:
  ^~~~

Quiet the warning by marking the expected switch fall through.

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