LoRa/SX126x: adjust Rx window offset and duration

Those timings open Rx time windows covering 99.7% of the one expected
by the antenna while allowing 3ms for the Rx subsystem to start listening.

embassy-lora/src/sx126x/mod.rs

@@ -55,10 +55,10 @@ where
     BUS: Error + Format + 'static,
 {
     fn get_rx_window_offset_ms(&self) -> i32 {
-        -500
+        -3
     }
     fn get_rx_window_duration_ms(&self) -> u32 {
-        800
+        1003
     }
 }