hw/sd/sdcard: Update the SDState documentation

Add more descriptive comments to keep a clear separation
between static property vs runtime changeable.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20200630133912.9428-12-f4bug@amsat.org>
diff --git a/hw/sd/sd.c b/hw/sd/sd.c
index c3febed..9b53053 100644
--- a/hw/sd/sd.c
+++ b/hw/sd/sd.c
@@ -102,11 +102,14 @@
     uint32_t card_status;
     uint8_t sd_status[64];
 
-    /* Configurable properties */
+    /* Static properties */
+
     uint8_t spec_version;
     BlockBackend *blk;
     bool spi;
 
+    /* Runtime changeables */
+
     uint32_t mode;    /* current card mode, one of SDCardModes */
     int32_t state;    /* current card state, one of SDCardStates */
     uint32_t vhs;