sdcard: run in slow mode (400 kHz during initialization)

Match the spec by running in slow mode (400 kHz) until after CMD0 is
accepted.

fw/sdcard.c

@@ -702,17 +702,17 @@ DSTATUS disk_initialize(BYTE drive)
 
     sdcard_led_on();
 
-    /* This doesn't work for MMC, but speeds up debugging */
-#undef SD_SLOW
-#define SD_SLOW SD_20MHZ
-
     /* Allow 4 retries in case the card is in a funky state */
     i = 4;
     while (1) {
-	/* Generate 256 clock cycles in slow mode, with CS# high */
+	/*
+	 * Generate 256 clock cycles in slow mode, with CS# high.
+	 * Use byte writes to reduce the time the CPU is held blocking
+	 * interrupts. (Todo: polling loop?)
+	 */
 	sd_set_mode(SD_SLOW, false);
-	for (j = 0; j < 8; j++)
-	    sd_writel(~0, SD_GO32);
+	for (j = 0; j < 32; j++)
+	    sd_writeb(~0, SD_GO8);
 
 	/* Assert CS# and send reset command */
 	sd_set_mode(SD_SLOW, true);