uhd: Add X300 sample timing for 4 SPS

Previously only 4/1 and 1/1 configurations Tx/Tx samples-per-symbol
were supported.

Change-Id: I9153171fe3af95e1cb0d9d35a9287dfde155d184
diff --git a/Transceiver52M/UHDDevice.cpp b/Transceiver52M/UHDDevice.cpp
index a84fe4b..0a640f9 100644
--- a/Transceiver52M/UHDDevice.cpp
+++ b/Transceiver52M/UHDDevice.cpp
@@ -121,6 +121,7 @@
 	{ USRP2, 4, 4, 4.6080e-5, "N2XX 4 SPS" },
 	{ B200,  4, 4, B2XX_TIMING_4_4SPS, "B200 4 SPS" },
 	{ B210,  4, 4, B2XX_TIMING_4_4SPS, "B210 4 SPS" },
+	{ X3XX,  4, 4, 5.6567e-5, "X3XX 4 SPS"},
 	{ UMTRX, 4, 4, 5.1503e-5, "UmTRX 4 SPS" },
 	{ LIMESDR, 4, 4, 16.5/GSMRATE, "STREAM/LimeSDR (4 SPS TX/RX)" },
 };