uhd: Use internal UHD tick conversions

UHD handles built in tick and floating point timestamp conversion
since version 003.005.004. This removes the need for separate UHD
timespec to tick conversion.

Signed-off-by: Tom Tsou <tom.tsou@ettus.com>
diff --git a/configure.ac b/configure.ac
index 0e35e36..6c6ca2e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -99,7 +99,7 @@
 AS_IF([test "x$with_usrp1" != "xyes"],[
     PKG_CHECK_MODULES(UHD, uhd >= 003.009,
         [AC_DEFINE(USE_UHD_3_9, 1, UHD version 3.9.0 or higher)],
-        [PKG_CHECK_MODULES(UHD, uhd >= 003.004.000)]
+        [PKG_CHECK_MODULES(UHD, uhd >= 003.005.004)]
     )
     AC_DEFINE(USE_UHD, 1, All UHD versions)
 ])