[BRIDGE]: Round off STP perodic timers.

Peroidic STP timers don't have to be exact.  The hold timer runs at
1HZ, and the hello timer normally runs at 2HZ; save power by aligning
it them to next second.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>