Bluetooth: classic: Kconfig: set BT_RFCOMM_DLC_STACK_SIZE default 512

For the shell RFCOMM case, these is a stack overflow problem when do
disconnect, so need to increase the CONFIG_BT_RFCOMM_DLC_STACK_SIZE.
Set CONFIG_BT_RFCOMM_DLC_STACK_SIZE default 512.

Signed-off-by: Make Shi <make.shi@nxp.com>