Bluetooth: Mesh: Fix sequence number restoring

The logic for restoring the sequence number was flawed in that it
would not always cause an incremented sequence write upon the
transmission of the first packet. The reason the code didn't work is
that it assumed the stored value was a multiple of SEQ_STORE_RATE,
however since the sequence number is stored in a deferred fashion
that's not always true.

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