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>
diff --git a/drivers/spi/Kconfig b/drivers/spi/Kconfig
index 201746a..1d1e396 100644
--- a/drivers/spi/Kconfig
+++ b/drivers/spi/Kconfig
@@ -325,7 +325,7 @@
 #
 
 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