- refine SBUS RX code (http://www.multiwii.com/forum/viewtopic.php?f=8&t=4425&start=30#p45299)

git-svn-id: http://multiwii.googlecode.com/svn/trunk/MultiWii_shared@1648 02679b44-d973-9852-f2fa-63770883b36c