drm_hwcomposer: fix small class consistency issues

This also fixes a typo in an error message.

Change-Id: Ie6c369f5eb2496ac3ebb342dbdcdd70f3a176495
diff --git a/hwcomposer.cpp b/hwcomposer.cpp
index a7c9e43..fc421c3 100644
--- a/hwcomposer.cpp
+++ b/hwcomposer.cpp
@@ -191,9 +191,9 @@
   return *this;
 }
 
-hwc_drm_bo *DrmHwcBuffer::operator->() {
+const hwc_drm_bo *DrmHwcBuffer::operator->() const {
   if (importer_ == NULL) {
-    ALOGE("Access of none existent BO");
+    ALOGE("Access of non-existent BO");
     exit(1);
     return NULL;
   }