Bluetooth: Kconfig: Increase TX stack size for BT_CTLR && BT_LL_SW_SPLIT

Increase the default TX stack size for BT_CTLR && BT_LL_SW_SPLIT,
as we have seen applications/samples nearing and even reaching
the stack size, causing stack overflows. This is especially
true if CONFIG_FPU=y which takes 96 bytes of the TX stack.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>