catkin.bbclass: make ROS_BP composed from ROS_BPN and version

The new definition was the original intention, and the originally
wrong definition happened to slip in.
Initial commit and discussion about the intention is in commit
c6837ecbf8.

classes/catkin.bbclass

@@ -31,7 +31,7 @@ FILES_SOLIBSDEV = ""
 FILES_${PN} += "${libdir}/lib*${SOLIBSDEV}"
 
 ROS_BPN = "${@d.getVar('BPN', True).replace('-', '_')}"
-ROS_BP = "${BPN}-${PV}"
+ROS_BP = "${ROS_BPN}-${PV}"
 
 FILES_${PN} += "\
     ${prefix}/etc \