Fix use of uninitialized value

==1116== Syscall param ioctl(generic) points to uninitialised byte(s)
==1116==    at 0x4F168B7: ioctl (syscall-template.S:84)
==1116==    by 0x400BB0: transfer (in /usr/bin/spi_xfer)
==1116==    by 0x400EFF: main (in /usr/bin/spi_xfer)
==1116==  Address 0xfff000a34 is on thread 1's stack
==1116==  in frame #1, created by transfer (???:)

spidev-app/spi_xfer.c

@@ -56,9 +56,10 @@ void transfer(struct spidev *dev, struct transfer *tr)
 		return;
 	}
 
+	memset(&iotr, 0, sizeof(iotr));
 	memcpy(tx, &tr->data, tr->len);
 	memset(&tx[tr->len], 0x00, len - tr->len);
-	memset(rx, 0xff, len);
+	memset(rx, 0x00, len);
 
 	printf("TX: ");
 	for (i = 0; i < len; i++)