add 10ms sleep after stop

This seems to stop consumers that are doing quick back to back stop/start
(eg gqrx changing decode mode / filter bandwidth) from hanging the
device.

I now don't have any weird hangs on hackrf with gqrx/freebsd/libusb!

When things hang it isn't erroring out in any way; it just doesn't
start receive.  It doesn't look like a libusb issue; I'd have to get
some USB bus sniffing to see what's going on behind the scenes.

host/libhackrf/src/hackrf.c

@@ -25,6 +25,7 @@ ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSI
 
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 #include <libusb.h>
 
 #ifdef _WIN32
@@ -1688,6 +1689,7 @@ int ADDCALL hackrf_stop_rx(hackrf_device* device)
 		return result;
 	}
 	result = hackrf_set_transceiver_mode(device, HACKRF_TRANSCEIVER_MODE_OFF);
+	usleep(10 * 1000);
 	return result;
 }
 
@@ -1717,6 +1719,7 @@ int ADDCALL hackrf_stop_tx(hackrf_device* device)
 		return result;
 	}
 	result = hackrf_set_transceiver_mode(device, HACKRF_TRANSCEIVER_MODE_OFF);
+	usleep(10 * 1000);
 	return result;
 }