libsel4platsupport: pass correct timer interface to tsc_get_timer

libsel4platsupport/src/plat/pc99/timer.c

diff --git a/libsel4platsupport/src/plat/pc99/timer.c b/libsel4platsupport/src/plat/pc99/timer.c
index 84536bd..a10a422 100644
--- a/libsel4platsupport/src/plat/pc99/timer.c
+++ b/libsel4platsupport/src/plat/pc99/timer.c
@@ -75,6 +75,6 @@
         return NULL;
     }
 
-    return finish_get_tsc(timer, tsc_get_timer(timeout_timer));
+    return finish_get_tsc(timer, tsc_get_timer(timeout_timer->timer));
 }