net: ip: Increase RX stack size

RX stack overflow when dealing with TCP over Bluetooth/6lo setup:

RX [rx_stack] stack size 1200/1264 bytes unused 0 usage 1200/1200 (100 %)

Change-Id: I8d5e43faf93f675960e59d9340b97d1ce9fc082a
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>