lib: change default number of transfers to 15

So far we had 32 * 256KB which was a bit overkill,
15 are more than enough.

15 was chosen instead of 16 because at least on Linux
there seems to be a system-wide limit of 63 transfers
(when they are 256KB large), so 4 dongles can be used
on a single machine without lowering the default transfer
number.

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

src/rtl_tcp.c

@@ -92,7 +92,7 @@ void usage(void)
 		"\t[-f frequency to tune to [Hz]]\n"
 		"\t[-g gain (default: 0 for auto)]\n"
 		"\t[-s samplerate in Hz (default: 2048000 Hz)]\n"
-		"\t[-b number of buffers (default: 32, set by library)]\n"
+		"\t[-b number of buffers (default: 15, set by library)]\n"
 		"\t[-n max number of linked list buffers to keep (default: 500)]\n"
 		"\t[-d device index (default: 0)]\n"
 		"\t[-P ppm_error (default: 0)]\n");