nasm: Fix -MD operating mode

Previously -MD option didn't call for depend mode solely but
produced deps during active compilation phase allowing symbol
refereces processing as well.

Occasionally in commit 599a98 I made -MD option to handle deps
in a separate internal phase which caused problems like

 | error: symbol references not supported in preprocess-only mode

leading to problems in building applications with nasm. Fix it
moving old behaviour back.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

nasm.c

@@ -898,7 +898,7 @@ set_warning:
                 depend_emit_phony = true;
                 break;
             case 'D':
-                operating_mode = OP_DEPEND | OP_NORMAL;
+                operating_mode = OP_NORMAL;
                 depend_file = q;
                 advance = true;
                 break;