BT Mesh LPN max. poll timeout calculated incorrectly

[stefbo]

Describe the bug
Bluetooth Mesh LPN poll frequency solely determined by poll timeout starting value CONFIG_BT_MESH_LPN_INIT_POLL_TIMEOUT. Seems to be a regression from 3b4d58a. The macro POLL_TIMEOUT_MAX should use CONFIG_BT_MESH_LPN_POLL_TIMEOUT instead of CONFIG_BT_MESH_LPN_INIT_POLL_TIMEOUT. The described algorithm to grow the poll timeout implemented in function poll_timeout does not work with this implementation. The algorithm is described in the description of Kconfig value BT_MESH_LPN_INIT_POLL_TIMEOUT.

To Reproduce
Configure an LPN with a starting value of say 5s and a poll timeout value of say 100s.

Expected behavior
The poll frequency starts at below 5s and is increased continuously towards a value derived from 100s.

Impact
Any reasonable low-power configuration has to set CONFIG_BT_MESH_LPN_INIT_POLL_TIMEOUT to CONFIG_BT_MESH_LPN_POLL_TIMEOUT. Existing devices updating to the current version without having such a configuration may suffer from too frequent polling.

[LingaoM]

@stefbo Thanks.