The shifting of timehi by 16 bits to the left will be promoted to
a 32 bit signed int and then sign-extended to an u64. If the top bit
of timehi is set then all then all the upper bits of ns end up as also
being set because of the sign-extension. Fix this by making timehi and
timelo u64.  Also move the declaration of ns.

Detected by CoverityScan, CID#1465288 ("Unintended sign extension")

Fixes: c6fe0ad2 ("net: dsa: mv88e6xxx: add rx/tx timestamping support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>