mesh: Fixes LPN poll timeout gradually grow

`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.

It's a regression in 3b4d58a
Bluetooth: Mesh: optimize performance for lpn node

This is port of aea9adfc8968e9210a4f58ac493fdc074478527b

nimble/host/mesh/src/lpn.c

@@ -48,11 +48,13 @@
 
 #define POLL_TIMEOUT_INIT     (MYNEWT_VAL(BLE_MESH_LPN_INIT_POLL_TIMEOUT) * 100)
 
+#define POLL_TIMEOUT              (MYNEWT_VAL(BLE_MESH_LPN_RECV_DELAY) * 100)
+
 #define REQ_ATTEMPTS_MAX      6
 #define REQ_ATTEMPTS(lpn)     MIN(REQ_ATTEMPTS_MAX, \
-			  				  POLL_TIMEOUT_INIT / REQ_RETRY_DURATION(lpn))
+			  				  POLL_TIMEOUT / REQ_RETRY_DURATION(lpn))
 
-#define POLL_TIMEOUT_MAX(lpn) (POLL_TIMEOUT_INIT - \
+#define POLL_TIMEOUT_MAX(lpn) (POLL_TIMEOUT - \
 			  					  (REQ_ATTEMPTS(lpn) * REQ_RETRY_DURATION(lpn)))
 
 #define CLEAR_ATTEMPTS        3

nimble/host/mesh/syscfg.yml

@@ -436,6 +436,16 @@ syscfg.defs:
             a value of 300 means 30 seconds.
         value: 300
 
+    BLE_MESH_LPN_POLL_TIMEOUT:
+        description: >
+            PollTimeout timer is used to measure time between two
+            consecutive requests sent by the Low Power node. If no
+            requests are received by the Friend node before the
+            PollTimeout timer expires, then the friendship is considered
+            terminated. The value is in units of 100 milliseconds, so e.g.
+            a value of 300 means 30 seconds.
+        value: 300
+
     BLE_MESH_LPN_INIT_POLL_TIMEOUT:
         description: >
             The initial value of the PollTimeout timer when Friendship