Bluetooth: Mesh: Fix setting an upper limit on beacon interval

https://www.bluetooth.org/errata/errata_view.cfm?errata_id=9807

"If the computed Beacon Interval is less than 10 seconds, it should be
set to 10 seconds. If the computed Beacon Interval is greater than 600
seconds, it should be set to 600 seconds."

The lower limit is already covered by how frequently the delayed work
callback gets called, so we just need enforce the 600 second maximum.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>