Bluetooth: RFCOMM: Increase dlc stack size to 256

This analyzes the dlc stack using stack_analyze() which is coming
as 188 with BLUETOOTH_DEBUG_LOG off.

dlc stack (real size 320):     unused 68       usage 188 / 256 (73 %)

So increase the stack size to 256.

Change-Id: Ie5d5f267f4f618747551f0bfd0e05ffb47e0bb91
Signed-off-by: Jaganath Kanakkassery <jaganathx.kanakkassery@intel.com>