axi_ltc2387: Make adc_valid to represent the current sample

* Before, adc_valid was for the previous sample. This said that
   at the second rising edge of clk_gate - the first sample is valid,
   which is not true

 * Alongside with the software issue that will be solved, these fixes
   will make the first 2 samples to be with valid data, otherwise the
   user has to always keep in mind that the first 2 ones are invalid

Signed-off-by: Iulia Moldovan <iulia.moldovan@analog.com>

library/axi_ltc2387/axi_ltc2387_if.v

@@ -96,7 +96,8 @@ module axi_ltc2387_if #(
 
   // assignments
 
-  assign adc_valid = ~clk_gate_d[2] & clk_gate_d[1];
+  // adc_valid is 1 for the current sample that is sent
+  assign adc_valid = clk_gate_d[1] & ~clk_gate_d[0];
 
   always @(posedge clk) begin
     clk_gate_d <= {clk_gate_d[1:0], clk_gate};