add debug messages to rtlsdr_read/write_array

Signed-off-by: Steve Markgraf <steve@steve-m.de>

src/librtlsdr.c

@@ -279,6 +279,9 @@ int rtlsdr_read_array(rtlsdr_dev_t *dev, uint8_t block, uint16_t addr, uint8_t *
 
 	r = libusb_control_transfer(dev->devh, CTRL_IN, 0, addr, index, array, len, CTRL_TIMEOUT);
 
+	if (r < 0)
+		fprintf(stderr, "%s failed with %d\n", __FUNCTION__, r);
+
 	return r;
 }
 
@@ -289,6 +292,9 @@ int rtlsdr_write_array(rtlsdr_dev_t *dev, uint8_t block, uint16_t addr, uint8_t
 
 	r = libusb_control_transfer(dev->devh, CTRL_OUT, 0, addr, index, array, len, CTRL_TIMEOUT);
 
+	if (r < 0)
+		fprintf(stderr, "%s failed with %d\n", __FUNCTION__, r);
+
 	return r;
 }