RP2040: Fix RAW fallback mapping not working.

The SDIO CSD register apparently cannot be read after 4-bit
bus mode has been enabled. This caused the SD card code to report
sector count of 0, which broke RAW fallback mapping on RP2040.

Fixed by reading CSD register on initialization like GD32 code does it.

lib/ZuluSCSI_platform_RP2040/sd_card_sdio.cpp

@@ -13,6 +13,7 @@
 static uint32_t g_sdio_ocr; // Operating condition register from card
 static uint32_t g_sdio_rca; // Relative card address
 static cid_t g_sdio_cid;
+static csd_t g_sdio_csd;
 static int g_sdio_error_line;
 static sdio_status_t g_sdio_error;
 static uint32_t g_sdio_dma_buf[128];
@@ -119,6 +120,13 @@ bool SdioCard::begin(SdioConfig sdioConfig)
         return false;
     }
 
+    // Get CSD
+    if (!checkReturnOk(rp2040_sdio_command_R2(CMD9, g_sdio_rca, (uint8_t*)&g_sdio_csd)))
+    {
+        azdbg("SDIO failed to read CSD");
+        return false;
+    }
+
     // Select card
     if (!checkReturnOk(rp2040_sdio_command_R1(CMD7, g_sdio_rca, &reply)))
     {
@@ -173,7 +181,8 @@ bool SdioCard::readCID(cid_t* cid)
 
 bool SdioCard::readCSD(csd_t* csd)
 {
-    return checkReturnOk(rp2040_sdio_command_R2(CMD9, g_sdio_rca, (uint8_t*)csd)); // SEND_CSD
+    *csd = g_sdio_csd;
+    return true;
 }
 
 bool SdioCard::readOCR(uint32_t* ocr)
@@ -203,9 +212,7 @@ bool SdioCard::readStop()
 
 uint32_t SdioCard::sectorCount()
 {
-    csd_t csd;
-    readCSD(&csd);
-    return sdCardCapacity(&csd);
+    return sdCardCapacity(&g_sdio_csd);
 }
 
 uint32_t SdioCard::status()

src/ZuluSCSI_disk.cpp

@@ -85,9 +85,11 @@ public:
             m_israw = true;
             m_blockdev = SD.card();
 
-            if (m_endsector >= SD.card()->sectorCount())
+            uint32_t sectorCount = SD.card()->sectorCount();
+            if (m_endsector >= sectorCount)
             {
-                m_endsector = SD.card()->sectorCount() - 1;
+                azlog("Limiting RAW image mapping to SD card sector count: ", (int)sectorCount);
+                m_endsector = sectorCount - 1;
             }
         }
         else