spi/dw_spi: add return value to empty mrst_spi_debugfs_init()

As per the function signature.
Signed-off-by: George Shore <george@georgeshore.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/dw_spi.c

@@ -152,6 +152,7 @@ static void mrst_spi_debugfs_remove(struct dw_spi *dws)
 #else
 static inline int mrst_spi_debugfs_init(struct dw_spi *dws)
 {
+	return 0;
 }
 
 static inline void mrst_spi_debugfs_remove(struct dw_spi *dws)