mac80211: Mark a destination sequence number as valid when a PREQ is received.

If a PREQ frame is received giving us a fresher DSN than what we have, record
the new dsn and mark it as valid.  This patch fixes a bug in the setting of the
MESH_PATH_DSN_VALID flag.

Also, minor fix to coding style on that file.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>