assemble: reduce the priority of "mask not permitted" error

Per BR 75, the "mask not permitted" error is a little too persistent.

Fixes: #75 (hopefully)
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>

Author: H. Peter Anvin (Intel)

asm/assemble.c

@@ -30,9 +30,9 @@ enum match_result {
     MERR_OPSIZEINVAL,
     MERR_OPSIZEMISSING,
     MERR_OPSIZEMISMATCH,
+    MERR_MASKNOTHERE,
     MERR_BRNOTHERE,
     MERR_BRNUMMISMATCH,
-    MERR_MASKNOTHERE,
     MERR_DECONOTHERE,
     MERR_BADZU,
     MERR_MEMZU,