async: context: add implicit conversion to bus_t

Many existing sdbusplus library calls expect a `bus_t`.  Add an
implicit conversion to `bus_t&` so that a `context_t&` can be passed
into any function that formerly accepted a `bus_t&`.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I791bf88d86ba5761a4d2fa9bf811b6e794b097af
diff --git a/src/async/context.cpp b/src/async/context.cpp
index 5bb58ae..b324c6c 100644
--- a/src/async/context.cpp
+++ b/src/async/context.cpp
@@ -281,7 +281,7 @@
 
     // Get the bus' timeout.
     uint64_t to_usec = 0;
-    sd_bus_get_timeout(get_busp(ctx.get_bus()), &to_usec);
+    sd_bus_get_timeout(get_busp(ctx), &to_usec);
 
     if (to_usec == UINT64_MAX)
     {