Add func to get nbr of PMU event counters

Small bugfix to a print in driver

Change-Id: I277a441a9f65c647c338b92af5a908176929102c
diff --git a/src/ethosu_driver.c b/src/ethosu_driver.c
index 522f3e8..fedb2a1 100644
--- a/src/ethosu_driver.c
+++ b/src/ethosu_driver.c
@@ -285,7 +285,7 @@
         drv = drv->next;
     }
 
-    LOG_DEBUG("No NPU driver handle available.", drv);
+    LOG_WARN("No NPU driver handle available.");
 
     return NULL;
 }