mtd: spi: SPI_FLASH_MTD depends on MTD

It is already the case that all defconfigs with SPI_FLASH_MTD also
declare using MTD, but let's make this consistent and enforce it in
Kconfig. Most of the time SPI_FLASH_MTD is used in conjunction with
UBI, which already depends on MTD.

Suggested-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
diff --git a/drivers/mtd/spi/Kconfig b/drivers/mtd/spi/Kconfig
index d77f818..018e8c5 100644
--- a/drivers/mtd/spi/Kconfig
+++ b/drivers/mtd/spi/Kconfig
@@ -186,7 +186,7 @@
 
 config SPI_FLASH_MTD
 	bool "SPI Flash MTD support"
-	depends on SPI_FLASH
+	depends on SPI_FLASH && MTD
 	help
           Enable the MTD support for spi flash layer, this adapter is for
 	  translating mtd_read/mtd_write commands into spi_flash_read/write