m25p80: fix test on blk_pread() return value

commit 243e6f69c1 ("m25p80: Switch to byte-based block access")
replaced blk_read() calls with blk_pread() but return values are
different.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/block/m25p80.c

@@ -900,7 +900,7 @@ static int m25p80_init(SSISlave *ss)
         s->storage = blk_blockalign(s->blk, s->size);
 
         /* FIXME: Move to late init */
-        if (blk_pread(s->blk, 0, s->storage, s->size)) {
+        if (blk_pread(s->blk, 0, s->storage, s->size) != s->size) {
             fprintf(stderr, "Failed to initialize SPI flash!\n");
             return 1;
         }