Fix for perror modifying errno in nandwrite

Since perror returns void, I assumed it wouldn't modify errno. I was wrong. And since the change was so simple, I didn't test it. Mea culpa. :(

This patch fixes it.
I don't use perror if pwrite fails with EIO because it's not useful. The failure of lseek or the message "Erasing failed write ..." implicitly says so since it's the only way to reach that code. There is not such additional message for the MEMERASE error so I use a temporary variable instead.

@@ -585,8 +585,8 @@ int main(int argc, char * const argv[])

			off_t rewind_bytes;
			erase_info_t erase;

-			perror ("pwrite");
			if (errno != EIO) {
+				perror("pwrite");
				goto closeall;
			}

@@ -605,8 +605,9 @@ int main(int argc, char * const argv[])

			fprintf(stderr, "Erasing failed write from %08lx-%08lx\n",
				(long)erase.start, (long)erase.start+erase.length-1);
			if (ioctl(fd, MEMERASE, &erase) != 0) {
+				int errno_tmp = errno;
				perror("MEMERASE");
-				if (errno != EIO) {
+				if (errno_tmp != EIO) {
					goto closeall;
				}
			}