[nrf toup] Increased packet buffer pool size

Increased default packet buffer pool size, as it was decreased
some time ago and we can observe lack of available buffers
in the load tests.

Signed-off-by: Kamil Kasperczyk <[email protected]>

Author: kkasperczyk-no

config/nrfconnect/chip-module/Kconfig

@@ -456,7 +456,7 @@ config CHIP_USE_OPENTHREAD_ENDPOINT
 
 config CHIP_SYSTEM_PACKETBUFFER_POOL_SIZE
   int "Packet buffer pool size"
-  default 11
+  default 15
   help
     Total number of packet buffers allocated by the stack for internal pool.