Reverted the changes from r528 as 'ret' was set every time at socket

Reverted the changes from r528 as 'ret' was set every time at socket
creation time even in the unlikely case that these protocol checks fail.
Additionally it was worse in readability.
Updated the comment for the whole protocol checks _code block_.
Therefore this comment is not directly placed before the first check.
I hope this was enough explanation why i reverted r528 like this.
If not, we should discuss it outside the SVN.
Oliver

git-svn-id: svn://svn.berlios.de//socketcan/trunk@529 030b6a49-0b11-0410-94ab-b0dab22257f2