ppc/pnv: Fix check on block device before updating drive contents

Test is wrong and the backend can never updated. It could have led to
a QEMU crash but since the firmware deactivates flash access if a valid
layout is not detected, it went unnoticed.

Reported-by: Coverity CID 1465223
Fixes: 35dde5766211 ("ppc/pnv: Add a PNOR model")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20211102162905.762078-1-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

hw/ppc/pnv_pnor.c

@@ -36,7 +36,7 @@ static void pnv_pnor_update(PnvPnor *s, int offset, int size)
     int offset_end;
     int ret;
 
-    if (s->blk) {
+    if (!s->blk || !blk_is_writable(s->blk)) {
         return;
     }