Bluetooth: Mesh: Fix encoding/decoding Publish Retransmit value

The Model Publish Retransmit Interval is in units of 50ms and not 10ms
like the other transmit/retransmit states. Create dedicated macros for
the Publish Retransmit State and use them where appropriate.

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