sandbox: Fix comment for nomap_sysmem() function

This should say 'cast' rather than 'case', so fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/include/asm/io.h b/arch/sandbox/include/asm/io.h
index a23bd64..3d09170 100644
--- a/arch/sandbox/include/asm/io.h
+++ b/arch/sandbox/include/asm/io.h
@@ -235,7 +235,7 @@
  * nomap_sysmem() - pass through an address unchanged
  *
  * This is used to indicate an address which should NOT be mapped, e.g. in
- * SMBIOS tables. Using this function instead of a case shows that the sandbox
+ * SMBIOS tables. Using this function instead of a cast shows that the sandbox
  * conversion has been done
  */
 static inline void *nomap_sysmem(phys_addr_t paddr, unsigned long len)