emummc: correct id size and also set id if emupath is used

This corrects a truncation that was happening and also if `emupath` key is used to change emuMMC on the fly, it now uses the path as id instead of 0.

bootloader/storage/emummc.h

@@ -37,7 +37,7 @@ typedef struct _emummc_cfg_t
 {
 	int   enabled;
 	u64   sector;
-	u16   id;
+	u32   id;
 	char *path;
 	char *nintendo_path;
 	// Internal.