Bluetooth: Mesh: Fix (re)transmit interval handling

The adv_send() function was incorrectly decoding the 5-bit value (it
was using it directly as milliseconds), which effectively lead to the
code always picking the controller's minimum supported interval.

Fix this issue, but do it by simplifying the (re)transmission state
tracking so that the state is always stored in the original "packed"
8-bit value, where 5 bits are reserved for the interval, and 3 for the
count.

Fixes #7972

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