fomu-flash: exit nonzero on write error

Signed-off-by: Sean Cross <sean@xobs.io>

fomu-flash.c

@@ -468,7 +468,7 @@ int main(int argc, char **argv) {
             break;
         }
         close(fd);
-        spiWrite(spi, 0, bfr, stat.st_size);
+        ret += spiWrite(spi, 0, bfr, stat.st_size);
         break;
     }