mac80211: fix erroneous clearing of MESH_PATH_SN_VALID flag

When a PREQ or PREP is received from an intermediate node, it contains
useful information for path selection but it doesn't include the
originator's sequence number.   Therefore, when updating the mesh path
to that intermediate node, we should not set the MESH_PATH_SN_VALID
flag.  BUT, if the flag is set, it should not be unset as we might have
received a valid sequence number for that intermediate node in the past.

This issue was reported, fixed and tested by Ya Bo (游波) and Pedro
Larbig (ASPj).

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