This should resolve http://bugzilla.kernel.org/show_bug.cgi?id=5519

The current feature computation loses bits that it doesn't know about,
resulting in an inability to add VLANs and possibly other havoc.
Rewrote function to preserve bits it doesn't know about, remove an
unneeded state variable, and simplify the code.
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>