mmc: Convert hs400_tuning flag from u8 to bool

This hs400_tuning is a flag, make it bool. No functional change.
This will be useful in the following patch, which adds another
more generic flag, where the compiler can better use the space
now reserved for the u8 to store more flags in it.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/include/mmc.h b/include/mmc.h
index 8df1413..a4584f8 100644
--- a/include/mmc.h
+++ b/include/mmc.h
@@ -736,7 +736,7 @@
 				  * accessing the boot partitions
 				  */
 	u32 quirks;
-	u8 hs400_tuning;
+	bool hs400_tuning:1;
 
 	enum bus_mode user_speed_mode; /* input speed mode from user */
 };