Implement !CONFIG_OPTIONROMS support for CSM

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/src/csm.c b/src/csm.c
index 45ca941..6e48bc6 100644
--- a/src/csm.c
+++ b/src/csm.c
@@ -187,6 +187,11 @@
     struct rom_header *rom;
     u16 bdf;
 
+    if (!CONFIG_OPTIONROMS) {
+        regs->ax = 1;
+        return;
+    }
+
     dprintf(3, "Legacy16DispatchOprom rom %p\n", table);
 
     dprintf(3, "OpromSegment   %04x\n", table->OpromSegment);