spi/dw_spi: Allow dw_spi.c to be a module

Signed-off-by: Jean-Hugues Deschenes <jean-hugues.deschenes@octasic.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/Kconfig

@@ -325,7 +325,7 @@ config SPI_NUC900
 #
 
 config SPI_DESIGNWARE
-	bool "DesignWare SPI controller core support"
+	tristate "DesignWare SPI controller core support"
 	depends on SPI_MASTER
 	help
 	  general driver for SPI controller core from DesignWare